SNS Organic Chemistry - Cliff's Flashcards Preview

DAT > SNS Organic Chemistry - Cliff's > Flashcards

Flashcards in SNS Organic Chemistry - Cliff's Deck (71):
1

Hybrid orbital number rule

= ∑ (σ bonds) - unpaired electrons

2 = sp

3 = sp2

4 = sp3

For example, carbon of ethene, = 3 - 0 = 3 = sp2 hybridisation

2

Alkanes

Synthesis

  1. Catalytic reduction of an alkene
  2. Wurtz reaction - alkyl halide + Na
  3. Alkyl halide via Grignard reaction - alkyl halide + Mg > Grignard reagent (MgCl group). Grignard reagent + H2O > alkane
  4. Reduction of alkyl halides - alkyl halide + Zn + HCl > alkane

3

Alkanes

Reasons for Lack of Reactivity

  1. Carbon-carbon and carbon-hydrogen single bonds are very strong due to good orbital overlap.
  2. C-H bonds make alkane molecules neither acidic nor basic because the electronegativity of both elements is very similar. This similarity gives the carbonhydrogen bond little polarity, and without polarity, proton loss is difficult. Thus, alkanes make poor acids. Likewise, a lack of nonbonded electron pairs on either the C or H atoms makes alkanes poor bases.

However, under proper conditions, alkanes can react with halogens and oxygen.

4

Alkenes

Synthesis

Dehydration of Alcohols

  • A molecule of water is eliminated from an alcohol molecule by heating the alcohol in the presence of a strong mineral acid.
  • A double bond forms between the adjacent carbon atoms that lost the hydrogen ion and hydroxide group.

5

Alkenes

Synthesis

Dehydration of Alcohols

Mechanism

  1. Protonation of the alcohol - simple acid-base reaction, which results in the formation of an oxonium ion, a positively charged oxygen atom.

  2. Dissociation of the oxonium ion - produces water and a carbocation (positively charged carbon atom and an unstable intermediate).

  3. Deprotonation of the carbocation -  positively charged end carbon of the carbocation attracts the electrons in the overlap region that bond it to the adjacent a carbon. This electron movement makes the α carbon slightly positive, which in turn attracts the electrons in the overlap regions of all other atoms bonded to it. This results in the hydrogen on the α carbon becoming very slightly acidic and capable of being removed as a proton in an acid-base reaction.

6

Zaitsev rule

  • It may be possible in some instances to create a double bond through an alcohol dehydration reaction in which hydrogen atoms are lost from two different carbons on the carbocation.
  • Major product is always the more highly substituted alkene (alkene with the greater number of substituents on the carbon atoms of the double bond) - an observation called the Zaitsev rule.
  • Thus, in the dehydration reaction of 2-butanol, 1- and 2-butene are formed.
  • The Zaitsev rule predicts that the major product is 2-butene. Notice that each carbon atom involved in the double bond of 2-butene has one methyl group attached to it. In the case of 1-butene, one carbon atom of the double bond has one substituent (the ethyl group), while the other carbon atom has no substituents.
     

7

Carbocation rearrangement

  • The carbocation in an alcohol dehydration may undergo rearrangement to form more stable arrangements.
  • Dehydration of 2-methyl-3-pentanol, for example, leads to the production of three alkenes.
  • The mechanism for the reaction shows that the extra compound formation is due to rearrangement of the carbocation intermediate.The 2-methyl-1-pentene molecule is formed via rearrangement of the intermediate carbocation.
  • The movement of a hydride ion (H:-) leads to the formation of a more stable carbocation (secondary to tertiary).

8

The inductive effect

  • Alkyl groups theoretically have the ability to “push” electrons away from themselves.
  • The greater the number of alkyl groups “pushing” electrons toward a positively charged carbon atom, the more stable the intermediate carbocation will be.
  • This increase in stability is due to the delocalization of charge density. A charge on an atom creates a stress on that atom. The more the stress is spread over the molecule, the smaller the charge density becomes on any one atom, reducing the stress. This lessening of stress makes the ion more stable.
  • Thus, tertiary carbocations, with three alkyl groups on which to delocalize the positive charge, are more stable than secondary carbocations, which have only two alkyl groups on which to delocalize the positive charge. For the same reason, secondary carbocations are more stable than primary carbocations.

9

Alkenes

Synthesis

 Dehydrohalogenation of alkyl halides

  • . Another β elimination reaction
  • Involves the loss of a hydrogen and a halide from an alkyl halide (RX).
  • Normally accomplished by reacting the alkyl halide with a strong base, such as sodium ethoxide.
  • This reaction also follows the Zaitsev rule, so in the reaction of 2-chlorobutane with sodium ethoxide, the major product is 2-butene.

10

Alkenes

Synthesis

Dehydrohalogenation

Mechanism

  1. A strong base removes a slightly acidic hydrogen proton from the alkyl halide via an acid-base reaction.

  2. The electrons from the broken hydrogen-carbon bond are attracted toward the slightly positive carbon atom attached to the chlorine atom.

  3. As these electrons approach the second carbon, the halogen atom breaks free, leading to the formation of the double bond.

11

Alkenes

Synthesis

Dehalogenation

  • Vicinal dihalides, which are alkane molecules that contain two halogen atoms on adjacent carbon atoms, can form alkenes upon reaction with zinc.

12

Alkenes

Synthesis

  1. Dehydration of alcohols
  2. Dehydrohalogenation
  3. Dehalogenation

13

Alkenes

Reactions

A-G

  1. Addition of carbenes
  2. Catalytic addition of hydrogen
  3. Electrophilic addition
  4. Epoxide reactions

 

14

Alkenes

Reactions

H-Z

  1. Halogenation
  2. Hydrohalogenation
  3. Hydration
  4. Hydroboration
  5. Oxidation and cleavage
  6. Polymerisation

15


Alkenes

Reactions

Electrophilic Addition

  •  The most common reactions of the alkenes are additions across the double bond to form saturated molecules.
  • Such reactions are represented by the following general equation, where X and Y represent elements in a compound that are capable of being added across the π-bond system of an alkene to form a substituted alkane

X-Y + H2C=CH2 → XCH2CH2Y

16

Alkenes

Reactions

Halogenation

  • Addition of halogen atoms to a π-bond system. For example, the addition of bromine to ethene produces the substituted alkane 1,2-dibromoethane.
  • Proceeds via a trans addition, but because of the free rotation possible around the single bond of the resulting alkane, a trans product cannot be isolated.
  • If, however, the original alkene structure possesses restricted rotation due to a factor other than a double bond, a trans-addition product can be isolated.
  • For instance, ring structures possess restricted rotation. In a ring structure, the carbon backbone is arranged so there is no beginning or ending carbon atom. If cyclohexene, a six-carbon ring that has one double bond, is halogenated, the resulting cycloalkane is trans substituted.

17

Alkenes

Reactions

Halogenation

Mechanism

  1. Alkenes and halogens are nonpolar molecules. However, both types of molecules, under proper conditions, can undergo induced-dipole formation, which leads to the generation of forces of attraction between the molecules.
  2. The bromoethyl carbocation that forms mid reaction in this example is often internally stabilized by cyclization into a three-membered ring containing a positively charged bromine atom (bromonium ion). This intermediate is more stable than the corresponding linear carbocation because all the atoms have a complete octet of electrons.
  3. The bromonium ion shares the electrons in the carbon-bromine covalent bond unevenly, with the overlap region being closer to the more electronegative bromine. This generates a partial positive charge (δ+) on the carbon atoms of the ring. The charge delocalization stabilizes the ring structure, and the resulting partial positive charges on the carbon atoms attract the nucleophilic bromide ion.
  4. The second bromide ion must approach a partially positive carbon atom from the side of the carbocation opposite where the bromonium ion attached. The reason for this is that the bromonium ion blocks access to the carbon atoms along an entire side, due to bond formation with the two carbon atoms. Such blocking is referred to as steric hindrance. Because of steric hindrance, only a trans addition is possible.

18

Alkenes

Reactions

Hydrohalogenation

Markovnikov

  • Unlike halogens, hydrogen halides are polarized molecules, which easily form ions. Hydrogen halides also add to alkenes by electrophilic addition.
  • The addition of hydrogen halides to asymmetrically substituted alkenes leads to two products
  • The major product is predicted by the Markovnikov rule, which states that when a hydrogen halide is added to an asymmetrically substituted alkene, the major product results from the addition of the hydrogen atom to the double-bonded carbon that is attached to more hydrogen atoms, while the halide ion adds to the other double-bonded carbon. This arrangement creates a more stable carbocation intermediate.

19

Alkenes

Reactions

Hydrohalogenation

Mechanism

Markovnikov

  1. The first step in the addition of a hydrogen halide to an alkene is the dissociation of the hydrogen halide.
  2. The H+ ion is attracted to the π-bond electrons of the alkene, which forms a π complex.
  3. The π complex then breaks, creating a σ single bond between one carbon of the double-bonded pair and the hydrogen. The carbon atom that loses a share of the π bond then becomes a carbocation. In asymmetrically substituted alkenes, two different carbocations are possible. The major product is generated from the more stable carbocation, while the minor product forms from the less stable one.


Thus, in the reaction between propane and HBr, the major product is 2-bromopropane.

20


Alkenes

Reactions

Hydrohalogenation

Anti-Markovnikov

  • The hydrogen atom of the hydrogen halide adds to the carbon of the double bond that is bonded to fewer hydrogen atoms.
  • For this to result, the reaction must proceed by a noncarbocation intermediate; thus in the presence of peroxide, the reaction proceeds via a free-radical mechanism, with the major product being generated from the more stable free radical.

21

Alkenes

Reactions

Hydrohalogenation

Antti-Markovnikov

Mechanism

  1. The mechanism for this reaction starts with the generation of a bromine free radical by the reaction of hydrogen bromide with peroxide.
  2. The bromine free radical adds to the alkene, forming a more stable carbon free radical.
  3. The secondary free radical (Br bount to terminal carbon) is more stable than the primary free radical because the secondary molecule is better able to delocalize the stress placed on the carbon atom by the free-radical electron. The major product then forms from the intermediates by reacting with hydrogen bromide.

22

Alkenes

Reactions

Hydration

  • The addition of water to an alkene in the presence of a catalytic amount of strong acid leads to the formation of alcohols (hydroxy-alkanes).
  • This reaction proceeds via a standard carbocation mechanism and follows the Markovnikov rule.

23

Alkenes

Reactions

Hydration

Mechanism

  1. The hydrogen ion is attracted to the π bond, which breaks to form a σ bond with one of the double-bonded carbons. The second carbon of the original double-bonded carbons becomes a carbocation.
  2. An acid-base reaction occurs between the water molecule and the carbocation, forming an oxonium ion.
  3. The oxonium ion stabilizes by losing a hydrogen ion, with the resulting formation of an alcohol

24

Alkenes

Reaction

Hydroboration Oxidation

  • Water can be added to an alkene in such a way that the major product is not that predicted by the Markovnikov rule. An example of such a reaction is the indirect addition of water to an alkene via a hydroboration-oxidation reaction.
  • In this reaction, a disubstituted boron hydride is added across the carbon-carbon double bond of an alkene.
  • The resulting organoborane compound is oxidized to an alcohol by reaction with hydrogen peroxide in a basic media, such as aqueous sodium hydroxide solution.
  • No carbocation intermediate forms during this reaction. Although the elements of water are added to an alkene, water is not a reactant; the hydrogen comes from a boron hydride molecule, and the hydroxide group comes from a peroxide molecule.

25

Alkenes

Reactions

Hydroboration Oxidation

Mechanism

  1. The first step in the hydroboration mechanism is the formation of the organoborane molecule from the alkene.
  2. The alkylborane then undergoes a three-stage oxidation reaction to form the alcohol

26

Alkenes

Reactions

Hydroboration Oxidation

Alkene

  • The first step in the hydroboration mechanism is the formation of the organoborane molecule from the alkene.
  • This reaction occurs rapidly.
  • The boron atom generally bonds to the less substituted, and thus less sterically hindered, carbon.
  1. This first step proceeds via a reaction between the disubstituted organoborane and the π bond of the alkene
  2. his is followed by formation of a C−H bond via a four-center interaction. A four-center interaction is a reaction in which bonds between four atoms are created and broken simultaneously.

27

Alkenes

Reactions

Hydroboration Oxidation

Alkylborane

 

  • The alkylborane then undergoes a three-stage oxidation reaction to form the alcohol:
  1. A hydroperoxide anion, formed by the reaction of a hydroxide ion with a peroxide molecule, adds to the electron-deficient boron atom
  2. This intermediate is unstable and rearranges, losing a hydroxide ion to form a borate ester.
  3. The borate ester then reacts with alkaline hydrogen peroxide to produce a trialkyl borate.
  4. Finally, the trialkyl borate is hydrolyzed (which means split by the elements of water) to alcohols and a borate ion by the aqueous hydroxide ion

28

Alkenes

Reactions

Catalytic Addition of Hydrogen

Hydrogenation is the addition of hydrogen to an alkene. Although this reaction is exothermic, it is very slow. The addition of a metal catalyst, such as platinum, palladium, nickel, or rhodium, greatly increases the reaction rate. Although this reaction seems simple, it is a highly complex addition. The reaction takes place in four steps.

29

Alkenes

Reactions

Addition of Hydrogen

Mechanism

  1. A hydrogen molecule reacts with the metal catalyst, breaking the σ bond between the hydrogen atoms and creating weak hydrogen-metal bonds.
  2. Next, the π bond of an alkene molecule contacts the metal catalyst.
  3. The π bond is destroyed and two weak carbon-metal single bonds are created.
  4. Finally, the weakly bound hydrogen atoms transfer one at a time from the catalyst surface to the carbon atoms of the former alkene molecule, forming an alkane. Upon formation of the two new carbon-hydrogen bonds, the alkane molecule can move away from the catalyst.

Because both of the added hydrogen atoms were bound to the surface of the catalyst, they normally approach the alkene molecule from the same side, or face. This approach of hydrogen atoms to the same face of an alkene molecule is called a syn addition.

When hydrogen atoms approach alkene molecules from opposite sides, the reaction is called an anti addition. Anti addition most likely occurs when double-bond isomerization occurs more rapidly than the catalytic addition of the second hydrogen in the hydrogenation.

30

Alkenes

Reactions

Addition of Carbenes

  • Carbenes are intermediates of the general formula R2C:.
  • In this configuration, the carbon atom possesses only a sextet of electrons, and is therefore highly reactive and electrophilic.
  • Carbenes are generally prepared by reacting a haloform, such as chloroform, with a strong base, such as sodium ethoxide.
  • Carbene (H2C:), however, is prepared by exposing diazomethane to ultraviolet light.

31

Alkenes

Reactions

Addition of Carbenes

Mechanism

  • Due to the high reactivity of carbenes, they cannot be isolated.
  • All carbene reactions are run by generating the carbene “in situ,” that is, generating the carbene in the presence of a reagent with which it will immediately react.
  • Alkenes, which are ready sources of electrons, are such reagents. When alkenes react with carbenes, three-membered rings are formed.

The insertion of a carbene into a π-bond system is the most common way of preparing cyclopropanes. The addition of the methylene unit, CH2, to the carbon-carbon double bond of the alkene is a syn addition.

32

Alkenes

Addition of Carbenes

Carbenoids

  • Some chemicals, namely the carbenoids, behave like carbenes, even though they are not.
  • The most common carbenoid is the Simmons-Smith reagent, a mixture of iodomethane and a zinc-copper couple. This reagent also reacts with alkenes to form a cyclopropane ring.
  • The mechanisms of carbene and carbenoid reactions show the difference between the two.
  • The mechanism for a carbene reaction is a concerted process in which all bonds are broken and formed at one time.
  • The mechanism for the Simmons-Smith reaction also shows a concerted addition; however, a carbene is never formed.

33

Alkenes

Reactions

Epoxide Reactions

  • Alkenes are capable of reacting with oxygen in the presence of elemental silver to form a series of cyclic ethers called epoxides.
  • Epoxides are three-atom cyclic systems in which one of the atoms is oxygen. The simplest epoxide is epoxyethane (ethylene oxide).
  • Epoxyethane belongs to a class of chemicals called heterocyclic compounds.
  • These compounds are cyclic structures in which one (or more) of the ring atoms is a hetero atom, that is, an atom of an element other than carbon.
  • In the laboratory, epoxyethane is prepared by reacting 1-chloro-2-hydroxyethane with a base.

34

Alkenes

Reactions

Epoxide Reactions

Mechanism

  1. Starts with the base reacting with the acidic hydrogen of the OH group.
  2. The oxygen anion is then attracted to the carbon that is bonded to the chlorine atom. This carbon bears a strong partial positive charge due to the great differences in electronegativity between the carbon and chlorine atoms.


The oxygen atom must be located anti to the departing chlorine atom for the reaction to occur. The overall reaction is a syn addition.

A third method of preparing epoxyethane is by the reaction of an alkene with peroxy acids.

35

Alkenes

Reactions

Oxidation and Cleavage Reactions

  • Alkenes can easily be oxidized by potassium permanganate and other oxidizing agents.
  • What products form depend on the reaction conditions.
  • At cold temperatures with low concentrations of oxidizing reagents, alkenes tend to form glycols.

36

Alkenes

Reactions

Oxidation and Cleavage Reactions

Baeyer Test

  • Because potassium permanganate, which is purple, is reduced to manganese dioxide, which is a brown precipitate, any water-soluble compound that produces this color change when added to cold potassium permanganate must possess double or triple bonds.
  • This reaction involves syn addition, leading to a cis-glycol (a vicinal dihydroxy compound).
  • A cis-glycol can also be produced by reacting the alkene with osmium tetroxide, OsO4.
  • When more concentrated solutions of potassium permanganate and higher temperatures are employed, the glycol is further oxidized, leading to the formation of a mixture of ketones and carboxylic acids.

37

Alkenes

Reactions

Oxidation and Cleavage

Ozonolysis

  • Oxidation of alkenes by ozone leads to destruction of both the σ and π bonds of the double-bond system.
  • This cleavage of an alkene double bond, generally accomplished in good yield, is called ozonolysis.
  • The products of ozonolysis are aldehydes and ketones.
  • This reaction is often used to find the double bond in an alkene molecule.
  • For example, the isomers of C4H8 can be distinguished from one another via oxidative cleavage.
  • By identifying the products of the reaction, one isomer can be distinguished from another, and the position of the bonds in the original compound can be determined.

38

Alkenes

Reactions

Polymerisation

 

  • Polymerization reactions proceed via either cationic or free-radical mechanisms.
  • In both processes, π bonds are converted to σ bonds, and energy is liberated.

39

Alkenes

Reactions

Polymerisation

Cationic

  •     Cationic polymerization is less efficient than free-radical polymerization due to the caustic nature of cation-producing reagents.
  •     For example, the reaction of ethene with sulfuric acid.

   

40


Alkenes

Reaction

Polymerisation

Free Radical

  • The more effective free-radical polymerization can be initiated by oxygen or other free-radical compounds, such as peroxides.
  • The free-radical polymerization of ethene by an alkoxide radical is a typical reaction.
  • The reaction may end by one of two termination steps. One is the bonding of two free radicals and the other is the internal stabilization of the polymer by double-bond formation

41


Alkynes

Isomerism

  • Although alkynes possess restricted rotation due to the triple bond, they do not have stereoisomers like the alkenes because the bonding in a carbon-carbon triple bond is sp hybridized.
  • In sp hybridization, the maximum separation between the hybridized orbitals is 180°, so the molecule is linear.
  • Thus, the substituents on triple-bonded carbons are positioned in a straight line, and stereoisomers are impossible.

42

Alkynes

Polarity

 

  • Substituted alkynes have small dipole moments due to differences in electronegativity between the triple-bonded carbon atoms, which are sp hybridized, and the single-bonded carbon atoms, which are sp3 hybridized.
  • The sp-hybridized carbon atom, which possesses more s character than the sp3-hybridized carbon atom, is more electronegative in character.
  • The resulting asymmetrical electron distribution in the bond between such carbon atoms results in the generation of a dipole moment.

43

Alkynes

Acidity

  •  Terminal alkynes are weakly acidic. Exposure to a strong base, such as sodium amide, produces an acid-base reaction.
  • The acidity of a terminal alkyne is due to the high level of s character in the sp hybrid orbital, which bonds with the s orbital of the hydrogen atom to form a single covalent bond.
  • The high level of s character in an sp-hybridized carbon causes the overlap region of the σ bond to shift much closer to the carbon atom. This polarizes the bond, causing the hydrogen atom to become slightly positive. This slight positive charge makes the hydrogen atom a weak proton, which can be removed by a strong base.
  • The reaction that forms the acetylide ion is reversible. Thus, the base may not form an acid of greater strength than the starting alkyne by acceptance of the proton, or the newly formed conjugate acid will reprotonate the acetylide ion. The fact that stronger acids are capable of reprotonating the acetylide ion can be seen in its reaction with water.

44

Alkanes and Alkene

Acidity

  • In the case of alkanes and alkenes, the s character in the hybridized carbon bonds is less, resulting in fewer electronegative carbon atoms and a corresponding lesser shift toward those atoms in the overlap region of the σ bond.
  • The location of the overlap region makes the corresponding hydrogen atoms less electron deficient and thus less acidic.
  • In reality, the hydrogen atoms bonded to alkanes and alkenes can be removed as protons, but much stronger nonaqueous bases are necessary.

45

Alkynes

Synthesis

  1. Dehalogenation
  2. Dehydrohalogenation
  3. Ethyne preparation
  4. Substitution

46


Alkynes

Synthesis

Dehydrohalogenation

  • The loss of a hydrogen atom and a halogen atom from adjacent alkane carbon atoms leads to the formation of an alkene.
  • The loss of additional hydrogen and halogen atoms from the double-bonded carbon atoms leads to alkyne formation.
  • The halogen atoms may be located on the same carbon (a geminal dihalide) or on adjacent carbons (a vicinal dihalide).
  • During the second dehydrohalogenation step, certain conditions are necessary, namely high temperatures and an extremely strong basic solution.

47

Alkynes

Synthesis

Dehalogenation

Vicinal tetrahaloalkanes can be dehalogenated with zinc metal in an organometallic reaction to form alkynes.

48

Alkynes

Synthesis

Substitution

  • Larger alkynes can be generated by reacting an alkyl halide with an acetylide ion, which is generated from a shorter alkyne.
  • Because acetylide ions are bases, elimination reactions can occur, leading to the formation of an alkene from the alkyl halide.
  • Because substitution and elimination reactions proceed through the formation of a common intermediate, these two types of reactions always occur simultaneously.

49

Alkynes

Synthesis

Ethyne

  • Ethyne, which is commonly called acetylene, is the simplest alkyne.
  • Historically, it was prepared by reacting calcium carbide with water.
  • Today, ethyne is normally prepared by the pyrolysis of methane. In this procedure, a stream of methane gas is briefly heated to 1500°C in an airless chamber. Air must be excluded from the reaction or oxidation (combustion) will occur.

50

Alkynes

Reactions

The principal reaction of the alkynes is addition across the triple bond to form alkanes. These addition reactions are analogous to those of the alkenes.

  1. Halogenation
  2. Hydration
  3. Hydrogenation
  4. Hydrohalogenation
  5. Oxidation
  6. Polymerisation

51

Alkynes

Reactions

Hydrogenation

To Alkanes

  • Alkynes undergo catalytic hydrogenation with the same catalysts used in alkene hydrogenation: platinum, palladium, nickel, and rhodium.
  • The mechanism of alkyne hydrogenation is identical to that of the alkenes. Because the hydrogen is absorbed on the catalyst surface, it is supplied to the triple bond in a syn manner.
  • Hydrogenation proceeds in a stepwise fashion, forming an alkene first, which undergoes further hydrogenation to an alkane.
  • This reaction proceeds so smoothly that it is difficult, if not impossible, to stop the reaction at the alkene stage

52

Alkynes

Reactions

Hydrogenation

To Alkenes

  • By using palladium or nickel for the catalyst, the reaction can be used to isolate some alkenes. Greater yields of alkenes are possible with the use of poisoned catalysts, for example Lindlar
  • Alkynes can also be hydrogenated with sodium in liquid ammonia at low temperatures. This reaction is a chemical reduction rather than a catalytic reaction, so the hydrogen atoms are not attached to a surface, and they may approach an alkene from different directions, leading to the formation of trans alkenes.

53

Lindlar's Catalyst

  • Used to reduce alkynes to alkenes
  • Composed of finely divided palladium coated with quinoline and absorbed on calcium carbonate.
  • This treatment makes the palladium less receptive to hydrogen, so fewer hydrogen atoms are available to react.
  • When a catalyst is deactivated in such a manner, it is referred to as being poisoned.

54

Alkynes

Reactions

Halogenation

  • The addition of halogens to an alkyne proceeds in the same manner as halogen addition to alkenes.
  • The halogen atoms add to an alkyne molecule in a stepwise fashion, leading to the formation of the corresponding alkene, which undergoes further reaction to a tetrahaloalkane.
  • Unlike most hydrogenation reactions, it is possible to stop this reaction at the alkene stage by running it at temperatures slightly below 0°C.

55

Alkynes

Reactions

Hydrohalogenation

  • Hydrogen halides react with alkynes in the same manner as they do with alkenes.
  • Both steps in the addition, for example of Hx to ethyne, follow the Markovnikov rule. Thus, the addition of hydrogen bromide to 1-butyne gives 2-bromo-1-butene as the major product of the first step.
  • The reaction of 2-bromo-1-butene in the second step gives 2,2-dibromobutane as the major product.

56

Alkynes

Reactions

Hydration

  • The addition of the elements of water across the triple bond of an alkyne leads to the formation of aldehydes and ketones.
  • Water addition to terminal alkynes leads to the generation of aldehydes, while nonterminal alkynes and water generate ketones.
  • These products are produced by rearrangement of an unstable enol (vinyl alcohol) intermediate.
  • The term “enol” comes from the en in “alkene” and ol in “alcohol,” reflecting that one of the carbon atoms in vinyl alcohol has both a double bond (alkene) and an OH group (alcohol) attached to it.
  • A vinyl group is CH2=CH- and a vinyl alcohol is CH2=CH-OH

57

Alkynes

Reactions

Hydration

Mechanism

Water adds across the triple bond of an alkyne via a carbocation mechanism. Dilute mineral acid and mercury(II) ions are needed for the reaction to occur.

 

  1. The first step of the mechanism is an acid-base reaction between the mercury(II) ion (Hg2+) and the π system of the alkyne to form a π complex.
  2. The π complex is converted into a single bond between one or the other of the carbons of the triple bond and the mercury (II) ion, with the resulting generation of a carbocation.
  3. A molecule of water is attracted to the carbocation to form an oxonium ion.
  4. The oxonium ion loses a proton to stabilize itself.
  5. The vinyl alcohol precursor that results is converted into vinyl alcohol (enol) by reaction with a hydronium ion (H3O+).
  6. Vinyl alcohols (enols) are unstable intermediates, and they undergo rapid isomerization to form ketones. Such isomerization is called keto-enol tautomerism
  7. In a similar fashion, the less-stable intermediate generates an aldehyde

58

Alkynes

Reactions

Oxidation

  • Alkynes are oxidized by the same reagents that oxidize alkenes. Disubstituted alkynes react with potassium permanganate to yield vicinal diketones (Vic-diketones or 1,2-diketones) or, under more vigorous conditions, carboxylic acids.
  • Ozonolysis of an alkyne also leads to carboxylic acid formation.

59

Alkynes

Reactions

Polymerisation

Alkynes can be polymerized by both cationic and free-radical methods. The reactions and mechanisms are identical with those of the alkenes.

60

Cyclohydrocarbons

Reactivity, Stresses of Small Rings

Angle Strain

  • All cycloalkane ring carbon atoms are sp3 hybridized, requiring bond angles that must be tetrahedral, or approximately 110°.
  • However, three- and four-membered carbon rings are planar, so their bonding angles are 60° and 90°, respectively.
  • The small size of these bond angles compared to the tetrahedral angle means that the orbital overlap region cannot exist directly between two carbon atoms. Rather, the two carbons are located at a slight angle to the overlap region, an arrangement that creates a weaker, more reactive bond.
  • This type of bonding strain is called angle strain.
  • Five-membered rings have a bond angle of 108°, which is very close to the tetrahedral angle. As a result, this ring system possesses little angle strain.
  • Rings of six carbons or more bend and thus maintain the stable tetrahedral bonding angle.

61

Cyclohydrocarbons

Reactivity, Stresses of Small Rings

Torsional Strain

  • In both the chair and boat forms of cyclohexane, there is no angle strain; however, the boat form has another type of ring strain called torsional strain.
  • Torsional strain is caused by the interaction of hydrogen atoms or substituents that are bonded to either adjacent or nonadjacent carbon atoms and situated in an eclipsed fashion.
  • The boat form of cyclohexane has two forms:
  1. Caused by the interaction of atoms or groups that are eclipsed on adjacent carbons. It occurs between the four lower H atoms on the four carbons at the bottom of the boat form of cyclohexane.
  2. Caused by eclipsed atoms or groups on nonadjacent carbons. This occurs between the eclipsed H atoms of the two upper carbons of the boat form.
  • These two types account for the higher energy states of the eclipsed cycloalkanes compared to those with staggered arrangements. Because the chair form of cyclohexane does not have torsional strain, it is more stable and has a lower energy state than the boat form.

62

Cyclohydrocarbons

Synthesis

Cycloalkanes can be prepared by ring-cyclization reactions, such as a modified Wurtz reaction or a condensation reaction. Additionally, they can be prepared from cycloalkenes and cycloalkynes

  1. Modified Wurtz
  2. Dieckmann Condensation 
  3. Hydrogenation 

Cycloalkenes and cycloalkynes are normally prepared from cycloalkanes by ordinary alkene-forming reactions, such as dehydration, dehalogenation, and dehydrohalogenation.

63

Cyclohydrocarbons

Reactions

  • Due to angle strain, the bonds in three- and four-membered carbon rings are weak. Because of these weak bonds, cyclopropane and cyclobutane undergo reactions that are atypical of alkanes. For example, cyclopropane reacts with halogens dissolved in carbon tetra-chloride to form dihaloalkanes.
  • Under similar conditions, straight-chain propane does not react.
  • In general, cycloalkanes undergo the normal reactions of the aliphatic alkanes (the straight-chain and branched-chain alkanes). Thus, cyclopentane will react with halogens in ultraviolet light to form halosubstituted cycloalkanes.
  • Cycloalkenes and cycloalkynes undergo the ordinary addition reactions of alkenes and alkynes. Cyclopropene, cyclopropyne, cyclobutene, and cyclobutyne also undergo ring-opening reactions

64

Conjugated Dienes

  • A diene is a molecule that has two double bonds. If the molecule is also a hydrocarbon, it is called an alkadiene.
  • When the double bonds are separated from each other by two or more single bonds, they are called isolated double bonds. Isolated double bonds undergo normal alkene reactions, revealing that no interaction occurs between them.
  • If, however, the double bonds are separated by only one single bond, atypical reactions occur.
  • Such an arrangement is called a conjugated double-bond system.
  • The interaction between the two double bonds in conjugated dienes delocalizes the electron density and increases the stability of the molecule.
  • The simplest conjugated diene is 1,3-butadiene.

65

Conjugated Dienes

Stability of Conjugated Systems

  • A conjugated diene system produces a low-energy state.
  • The complete delocalization of the π system gives the single bond some double-bond character and explains why it is slightly shorter than expected.
  • Rotation around this single bond is also somewhat restricted because of the partial double-bond character of the bond and because of an increase in repulsion between groups attached to the terminal carbons.
  • The increase in repulsive forces is due to the shorter bond length, which brings the groups closer together.

66

Conjugated Dienes

Synthesis

  • Prepared from the same reactions that form ordinary alkenes.
  • The two most common methods are the dehydration of diols (dihydroxy alkanes) and the dehydrohalogenation of dihalides (dihaloalkanes).
  • The generation of either an isolated or conjugated system depends on the structure of the original reactants.
  • Vicinal diols, which have two hydroxyl groups on adjacent carbon atoms, and vicinal dihalides, which have halogen substituents on adjacent carbons, always become conjugated systems in elimination reactions.
  • Other reactant configurations can lead to products that include both conjugated and isolated systems.

67

Conjugated Dienes

Diels-Alder reaction

  • Cycloaddition reaction between a conjugated diene and an alkene.
  • Produces a 1,4-addition product.
  • A typical example is the reaction of 1,3-butadiene with maleic anhydride.
  • Favored by the presence of electron-withdrawing groups on the diene and electron-releasing groups on the dienophile, which is a group or bond that is attracted to a diene.

68

Conjugated Dienes

Diels-Alder reaction

Mechanism

  • Doesn't run via a carbocation intermediate. Instead proceeds by a pericyclic process, a mechanism of just one step, involving a cyclic redistribution of bonding electrons.
  • Simple alkenes and alkynes are not good dienophiles. A good dienophile generally has one or more electron-withdrawing carbonyl groups or cyano groups attached to it.
  • Electron-supplying groups on the diene make the electrons of the π system more available for reaction.

69

Conjugated Dienes

Diets-Alder Reactioh

Stereochemistry

  • The Diels-Alder reaction is very stereospecific.
  • The original stereochemistry of the diene and the dienophile are preserved during this syn-addition reaction.
  • Because the reaction is basically a concerted cyclization, the diene must react in the cis conformation.
  • If the diene is a ring structure, the Diels-Alder reaction produces a bicyclic ring system.

70

Conjugated Dienes

Diets-Alder Reaction

Bicyclic ring system

  • Two carbon rings that share common sides. The previous diagram shows w
  • What appears to be a cyclohexene ring with a carbon bridge connecting the third and sixth carbons in reality is a system with two five-membered rings, a cyclopentene ring and a cyclopentane ring, which are sharing two sides (the “carbon bridge”).

71

Decks in DAT Class (53):