What is an unsaturated hydrocarbon?
A hydrocarbon containing one or more carbon-carbon double or triple bonds.
What is the theoretical bond angle for a carbon-carbon double bond?
120 degrees (sp2 hybridization)
What causes deviations from the theoretical bond angle in alkenes?
Strain from nonbonded interactions created by groups bonded to the carbons of the double bond.
Describe the carbon-carbon double bond in an alkene using atomic orbitals
The double covalent bond consists of one sigma bond created from the overlap of two sp2 hybridized orbitals and a pi bond created from the parallel unhybridized p orbitals.
Why does a double bond restrict rotation?
The two carbon atoms and the four bonded atoms must lie in a plane for the unhybridized p orbitals to be parallel.
When is cis,trans system appropriate for naming an alkene?
When each carbon of the c=c bond bears a hydrogen
What are the four steps for naming a normal alkene?
- Number longest chain that contains double bond in the direction that gives the carbons of double bond the lowest number.
- Indicate double bond by the number of its first carbon
- Name substituent groups
- Put all the parts of the name together.
What is the IUPAC name for Methylene group?
What is the IUPAC name and structure for Vinyl group?
What is the IUPAC name for Allyl group?
In cis,trans alkenes, what does cis mean?
molecules in which the carbon atoms of the main chain are on the same side of the double bond
In cis,trans alkenes, what does trans mean?
molecules in which the carbon atoms of the main chain are on opposite side of the double bond
When using E,Z naming system, how do you assign priority?
Priority is assigned by atomic number of atoms bonded directly to the carbons participating in double bond (higher atomic number is higher priority). Priority assignment is made at the first point of difference between groups.
What E,Z configuration is used if the groups of higher priority are on the SAME side of the double bond?
Z (German: zusammen)
What E,Z configuratoin is used if the groups of higher priority are on OPPOSITE sides of the double bond?
E (German: entgegen)
How are carbon atoms of cycloalkenes numbered?
The carbon atoms of the ring double bond are numbered 1 and 2; the direction is chosen by giving the first substituent the smallest number.
Describe the relative stability of cis,trans isomers of cycloalkenes (three components to answer)
(1) Cis cycloalkenes are generally more stable than trans cycloalkenes; (2) the latter is only stable enough to exist with rings of 8 carbons or greater. (3) For rings greater than 11 carbons, trans cycloalkenes are more stable.
What causes the relative instability of the trans isomers of cycloalkenes?
The trans double bond causes a strong twisting of the ring which creates high ring strain.
In bycyclic alkenes, what position of the double bond causes the highest strain?
Double bond is placed at a bridge head carbon (a carbon shared by both rings)
How do you change the infix -en- for an alkene containing two or more double bonds?
-adien-, -atrien-, and so on
For an alkene containing “n” double bonds, how many stereoisomers are possible?
What is the only intermolecular forces between molecules of an alkene?
Alkenes of what number of carbons are gas at room temperature?
up to four carbons
What is significant about the bond between a sp3 hybridized carbon and a sp2 hybridized carbon?
The sigma bond formed between an sp3 and a sp2 overlap is stronger than an sp3 and sp3 overlap.
What causes the dipole moment in alkenes (2 components to answer)?
(1) An sp2 carbon is slightly more electronegative than an sp3 carbon because the increase in s character increases electron density. (2) This causes sp3-sp2 carbon bonds to be slightly polar with the negative end being at the sp2 carbon.
How does the dipole moment of alkenes affect relative melting points of stereoisomers (2 components to answer)?
(1) A stereoisomer that causes sp3-sp2 polarization to not cancel out (ex. cis-2-butene) will have a small dipole moment. (2) These stereoisomers will have a lower melting point because the dipole moment repulsion will make solid formation less stable.
What is an addition reaction?
A reaction in which two atoms or ions react with a double bond forming a compound with the two new groups bonded to the carbons of the original double bond.
What is a reaction intermediate?
A high-energy species, formed between two successive reaction steps, that lies in an energy minimum between the two transition states.
What is a concerted reaction?
A reaction that occurs in one step.
What is a carbocation?
An intermediate where carbon has three bonds (six valence electrons) and is positively charged.
What is the rate-determining step of a reaction?
The step in a multistep reaction that crosses the highest energy barrier (has the highest activation energy).
What is a carbon radical?
An intermediate where carbon has three bonds and one unpaired electron (7 electrons)
What is a carbanion?
An intermediate where carbon has three bonds and one lone pair of electrons (8 electrons – full octet) and is negatively charged.
Compare the lifetimes of transition states and reaction intermediates (2 components to answer)
(1) Transition states mostly have theoretical lifetimes and only exist on the time scale that it takes for a bond to vibrate (about one billionth of a second.) (2) Reaction intermediates have distinct measurable lifetimes although they may be extremely short due to their reactivity.
What is bond dissociation enthalpy?
The amount of energy required to break a bond into two radicals (i.e. split X-Y into X• and •Y)
How are bond strengths and bond dissociation enthalpies related?
They are the same in magnitude but opposite in sign (bond dissociation is positive because you are pumping in energy to break bond, bond strengths are negative)
Define an exothermic reaction in terms of bond strengths of products and reactants
A reaction is exothermic if stronger bonds are made in the product compared to those broken in the starting materials.
What is electron pushing?
The use of arrows in organic chemistry mechanisms to indicate the flow or movement of electrons.
What does a double-barbed arrow indicate in electron pushing?
Movement of an electron pair
What does a single-barbed arrow indicate in electron pushing?
Movement of a single electron (involved with radical reactions)
What is an electron source?
A bond or lone pair of electrons (usually an area of relatively high electron density) that will be the origin of a mechanism arrow.
What is an electron sink?
An atom that accepts a new bond or lone pair of electrons; the mechanism arrow points to the electron sink.
What is a nucleophile?
A molecule that serves as an electron source to donate a pair of electrons to form a new covalent bond. Equivalent to a Lewis base.
What is an electrophile?
An molecule that serves as an electron sink to accept a pair of electrons to form a new covalent bond. Equivalent to a Lewis acid.
What is the special term for a nucleophile in reactions in which a proton is transferred?
What are the three different situations that arrows in electron pushing can be classified?
- redistrubution of pi bonds and/or lone pairs
- formation of a new sigma bond from a lone pair or pi bond
- breaking a sigma bond to give a new lone pair or pi bond
What is a common step in a reaction mechanism when there is a nucleophile present in the solution as well as an electrophile suitable for reaction to occur?
Make a new bond between a nucleophile (source for an arrow) and an electrophile (sink for an arrow).
What is a common step in a reaction mechanism when there is no suitable nucleophile-electrophile or proton transfer reaction, but breaking a bond can create neutral molecules or relatively stable ions?
Break a bond so that relatively stable molecules or ions are created
What is a common step in a reaction mechanism when there is no suitable nucleophile-electrophile reaction, but there is a strong acid present or the molecule has a strongly basic functional group?
Add a proton (nucleophile is the source and Hydrogen [proton] is the sink)
What is a common step in a reaction mechanism when there is no suitable nucleophile-electrophile reaction, but the molecule has a strongly acidic proton or there is a strong base present?
Take a proton away (nucleophile is the source and Hydrogen [proton] is the sink)
What will occur to avoid hypervalence (overfilling valence) on an atom serving as an electron sink?
What is an electrophilic addition?
A type of addition reaction with alkenes in which an electrophilic species adds to a pi bond.
What are three characteristics that can make a molecule an electrophile?
- region of low electron density (full or partial positive charge)
- lack of an octet on an atom
- relatively weak bond to an atom that can depart as a stable ion or molecule
What makes molecular halogens electrophilic?
The X-X bond is weak compared to the one they form upon reaction, and the (X-) ion is relatively stable.
What is the product of a reaction between an alkene and a halohalide (HX)?
haloalkanes (alkyl halides)
What is a regioselective reaction?
A reaction in which one direction of bond forming or breaking occurs in preference to all other directions of bond forming or breaking.
What is Markovnikov’s rule?
In the addition of HX, H2O or ROH to an alkene, hydrogen adds to the carbon of the double bond having the greater number of hydrogens.
What are the two steps of the mechanism for Electrophilic Addition of HX to an alkene?
- Make a bond between nucleophile (pi bond) and an electrophile (hydrogen in HX) to form a carbocation intermediate
- Make a new bond between a nucleophile (bromide ion) and electrophile (carbocation)
Why is the pi bond considered relatively electron rich?
The electron density lies above and below the bond axis, not between the nuclei as in the case with sigma-bonding electron density.
How do you classify a carbocation as primary, secondary or tertiary?
The number of carbon atoms bonded to the carbon bearing the positive charge.
Are carbocations nucleophiles or electrophiles?
All carbocations are electrophiles as well as Lewis acids.
What is the predicted bond angle of a carbocation?
Approximately 120 degrees (sp2 hybridization with an empty unhybridized p orbital)
What explains the regioselectivity of the Electrophilic Addition of HX to an alkene?
Differences in the relative stability of the carbocation. The more stable carbocation is the one that is formed.
Explain Markovnikov’s rule in terms of carbocations (3 components to answer)
(1) Fewer carbon neighbors means there is less stabilization of the possible carbocation. (2) A carbon with more hydrogen bonds will have fewer carbon neighbors. (3) The hydrogen will attach to the carbon with more hydrogen bonds so that the carbocation can be formed on the carbon with more carbon neighbors.
Rank the carbocation stabilities from most stable to least stable
Tertiary > Secondary > Primary > Methyl
How does the inductive effect stabilize a carbocation? (2 components to answer)
(1) The inductive effect delocalizes the positive charge over the volume of space occupied by the entire ion, not just the positive carbon. (2) The greater the volume the charge is delocalized, the greater the stability.
How does hyperconjugation stabilize a carbocation?
hyperconjugation increases electron density on the positive carbon’s vacant 2p orbital through overlap (sharing of electrons) of an adjacent alkyl group’s sigma bond
What is hydration?
The addition of water
What is used as a catalyst for the addition of water on an alkene to form an alcohol?
Acid (H3O+). It is consumed in Step 1 but another is generated in Step 3.
What are the three steps of the mechanism for Acid-Catalyzed Hydration of an Alkene?
- Make a new bond between a nucleophile (pi bond) and an electrophile (H+) to form carbocation intermediate
- Make a new bond between a nucleophile (carbocation) and electrophile (H20)
- Take a proton away (hydrogen attached to oxonium on alkane joins with H2O and forms new H30+)
What is an oxonium ion?
An ion in which oxygen bears a positive charge
What is the simplest oxonium ion?
What is rearrangement?
A change in connectivity of the same atoms between starting materials and products.
What is a 1,2-shift?
A type of rearrangement in which an atom or group of atoms moves (with its bonding electrons) from one atom to an adjacent electron-deficient atom.
What is the driving force for rearrangement of carbocations?
Secondary carbocations rearrange to tertiary carbocations or more stable secondary carbocations because it is energetically favorable.
When the group that migrates in a 1,2-shift is a hydrogen, what is the name given to that hydrogen and why?
Hydride ion (H-) because it migrates with BOTH electrons of the covalent bond.
In acid-catalyzed hydration of an alkene, where is the hydrogen added in step 1?
Hydrogen adds to the carbon of the double bond with the greatest number of hydrogens (Markovnikov’s rule). This is in order to form the most stable carbocation.
What is the rate determining step of the acid-catalyzed hydration of an alkene?
Formation of a carbocation intermediate
What is the rate determining step of Electrophilic Addition of HX to an alkene?
Formation of a carbocation intermediate
What is anti stereoselectivity?
The addition of atoms or groups of atoms to opposite faces of a carbon-carbon double bond (i.e. one addition is above and one addition is below the double bond)
What are the two steps of the mechanism for the addition of Halogen (X-X) to an alkene?
- Make a new bond between a nucleophile (pi bond) and an electrophile (Halogen) to form a bridged halonium ion intermediate.
- Make a new bond between a nucleophile (halide ion) and electrophile (carbon from opposite side of bromonium ion)
What is a halonium ion
An ion containing a halogen with a positive charge
What causes a bridged halonium ion to form?
There are three contributing structures in the reaction intermediate: two carbocations (bromine attached to a single carbon) and a bromonium ion attached to both carbons. The true structure is thus a ring with bromine weakly attached to both carbons.
What is a bridged halonium ion?
A cyclic structure of which a halonium ion is a part.
Describe the structure of the initial product of the addition of a halogen to a cyclohexane and explain why
trans diaxial because on adjacent atoms of a cyclohexane ring, only AXIAL positions are anti and co planar.
In the X-X addition of an alkene mechanism, why is the second halogen addition anti-coplanar to the first halogen? (2 components to answer)
(1) The second halogen is unable to attack on the same side of the carbon as the first halogen because it is blocked by the bridged halonium ion. (2) Because they attack on opposite sides they are considered anti-coplanar.
What kind of stereoselectivity is seen in the addition of a halogen to an alkene?
What is a halohydrin?
A compound containing a halogen atom (X) and a hydroxyl group (OH) on adjacent carbons.
What kind of solvent is used in an addition reaction with an alkene to form a halohydrin?
What kind of solvent is used in an addition reaction with an alkene to form a dihaloalkane?
An inert solvent such as CH2Cl2 or CCl4
Describe the regioselectivity and stereoselectivity of the formation of a halohydrin from an alkene
Halogen and the hydroxyl group add anti to each other with X bonding to the less substituted carbon and OH bonding to the more substituted carbon.
What are the three steps of the mechanism for the halohydrin formation of an alkene?
- Make a new bond between a nucleophile (pi bond) and an electrophile (halogen) forming bridged halonium intermediate
- Make a new bond between a nucleophile (water) and an electrophile (more substituted carbon of the bridged halonium ion).
- Take a proton away (nucleophilic water molecule attacks electrophilic hydrogen on oxonium group forming new hydronium ion)
Why does the water molecule attack the more substituted carbon of the bridged halonium ion? (2 components to answer)
(1) A secondary carbocation (the more substituted carbon) has more contribution to the resonance hybrid because it is more stable. (2) The more substituted carbon will thus have more carbocation character which makes the nucleophile preferentially attack it.
How does the activation energy of the ring-opening transition state effect the regioselectivity of the water addition in halohydrin formation? (2 components to answer)
(1)The bond between the bridged halonium ion and the more substituted carbon is the longer of the two bonds. (2) The longer bond will have a lower activation energy, so the ring-opening transition state can be reached more easily by attack at the more substituted carbon.
What two factors explain the regioselectivity of halohydrin formation from an alkene?
- carbocation character
2. activation energy of the ring-opening transition state
What is the intermediate of the addition of a halogen to an alkene in an inert solvent?
A bridged (cyclic) halonium ion
What is the intermediate of the addition of a halogen to an alkene in water?
A bridged (cyclic) halonium ion
What is oxymercuration-reduction? (2 components to answer)
(1) A method for converting an alkene to an alcohol (Markovnikov addition of H-OH). (2) The alkene is treated with mercury (II) acetate followed by reduction with sodium borohydride.
What is an important difference in the products of oxymercuration-reduction and acid-catalyzed hydration and what does this indicate?
Oxymercuration-reduction occurs without rearrangement, indicating that at no time is a free carbocation intermediate formed.
What are the five steps of the mechanism for oxymercuration-reduction of an alkene?
- Break a bond to give stable ions (dissociation of mercury(II) acetate gives AcOHg+ and acetate anion)
- Make a new bond between a nucleophile (pi bond) and an electrophile (AcOHg+) forming cyclic mercurinium ion intermediate
- Make a new bond between a nucleophile (water) and an electrophile (more substituted carbon) (anti addition)
- Lose a proton (water molecule nucleophile attacks an electrophilic hydrogen on oxonium ion)
- Reduction of C-HgOAc to C-H by reaction with sodium borohydride (NaBH4) gives final product and metallic mercury
Explain why even though the -OH group and the Hg atom add with anti stereochemistry in the oxymercuration step, there is a mixture of syn and anti products
The reduction of C-HgOAc to C-H by sodium borohydride results in a homolytic cleavage of the C-Hg bond, forming a free radical, that can react with hydrogen on any face, creating a mixture of syn and anti products.
What is the reaction intermediate for the oxymercuration step of oxymercuration-reduction of an alkene?
A bridged (cyclic) mercurinium ion
What solvent is used in oxymercuration-reduction of an alkene?
What two (2) factors are used to explain the regioselectivity and stereoselectivity of oxymercuration of an alkene?
Same as hydride formation of an alkene.
- carbocation character
- activation energy of ring-opening transition state
What is hydroboration-oxidation?
A method for converting an alkene to an alcohol. The alkene is treated with borane (BH3) to give a trialkyl borane, which is then oxidized with alkaline hydrogen peroxide to give the alcohol.
What is non-Markovnikov hydration?
hydrogen is added to the more substituted carbon of the double bond and -OH to the less substituted carbon
describe the regiochemisry of hydroboration of an alkene
What hybridization does Boron use to bond with three other atoms and what are its expected bond angles?
sp2 hybridization and 120 degrees
Are trivalent compounds of Boron nucleophiles or electrophiles?
Electrophiles because of the vacant 2p orbital
What is the formal charge of Boron in a trivalent compound?
What is syn stereoselectivity?
The addition of atoms or groups of atoms to the same face of a carbon-carbon double bond (i.e. both above or both below double bond)
Describe the stereoselectivity of hydroboration of an alkene
syn stereoselective (syn addition)
Why is THF often added to BH3 in the reagents of a hydrboration reaction?
To prevent BH3 from reacting with itself due to its empty p orbital and forming B2H6 (a toxic gas that ingnites spontaneously in air)
What is the hydroperoxide ion?
The conjugate base of hydrogen peroxide
What is the two step mechanism of hydroboration to form a trialkylborane?
- 2p orbital of boron is coordinated with electron pair of pi bond.
- boron adds to the less substituted carbon of the alkene and hydrogen adds simultaneously to the same side of the double bond (concerted, syn addition)
* Repeat twice more until you have boron bonded to 3 alkanes
What effect explains the regioselectivity of the addition of BH3 to an alkene?
BH3 is sensitive to steric interaction so it avoids the cluttered, more highly substituted carbon.
What is the three step mechanism for the oxidation of a trialkyl borane by alkaline hydrogen peroxide?
- Make a new bond between a nucleophile (hydroperoxide ion) and an electrophile (boron atom of trialkylborane)
- 1,2 shift (R group on boron attaches to an adjacent oxygen and results in ejection of hydroxide ion). *This repeats twice, replacing all B-R bonds with B-O-R bonds
- Trialkylborate (RO)3B reacts with NaOH (R-O-B is replaced by R-OH) to give the alcohol and sodium borate (Na3BO3)
Describe the stereoselectivity of hydrogen peroxide oxidation of a trialkyl borane and what this indicates about the products of hydroboration-oxidation reactions
Hydrogen peroxide oxidation of a trialkylborane retains stereochemistry; the OH that replaces boron will be in whatever position boron was in relation to those groups. Boron had non-markovnikov syn addition to the alkene, so the OH will also be in this position.
What is oxidation?
The loss of electrons. Alternatively, either the loss of hydrogens, the gain of oxygens, or both.
What is reduction?
The gain of electrons. Alternatively, the gain of hydrogens, loss of oxygens, or both.
What is the common name for a 1,2-diol
Glycol or Vicinal diol (-OH groups on adjacent carbons)
What reagents can be used for cis glycol (syn addition) formation of an alkene?
oxidation with cold KMNO4 or Osmium tetroxide (OsO4) (certain transition metal oxides)
What accounts for the syn addition of Osmium tetroxide or Manganate when oxiding an alkene?
The intermediate contains a five-membered metal-containing ring bonded in a cis configuration formed by concerted bonding of an oxygen of the oxide to the pi electrons of the original alkene Treatment with a reducing agent such as sodium bisulfate (OsO4) or hydroxide (KMnO4) cleaves the metal-oxygen bond and replacement with a hydrogen results in -OH formation in the same cis configuration.
In a half reaction, how do you balance hydrogens in acidic conditions?
In half reactions, how do you balance oxygens in acidic conditions?
In half reactions, how do you balance hydrogens and oxygens in basic conditions?
Add H20 and OH-
What is a permanganate?
A compound containing the manganate (VII) ion
Is the manganate (VII) ion a reducing agent or an oxidizing agent?
Because manganese is in the +7 oxidation state, the permanganate ion is a strong oxidizing agent.
What reagents can be used for trans glycol (anti addition) formation of an alkene?
What is an epoxide?
A cyclic ether in which oxygen is one atom of a three-membered ring.
Describe, in general, how a glycol is formed from an alkene using peroxyacids.
- RCO3H (a peroxyacid) oxidizes alkene carbons forming an epixode ring structure.
- H+ ions protonate the oxygen in the epixode ring forming a bridged oxonium ion.
- A backside attack by H2O on a carbon opens up the ring, resulting in an -OH on one carbon and an -H2O on the other carbon.
- A proton from H20 transfers to solvent leaving an -OH.
Describe, in general, the free radical addition of HBr to an Alkene.
- Peroxide decomposes forming a radical and hydroxide.
- Peroxide radical reacts with HBr forming a Bromine radical.
- Alkene pi bond attack Bromine radical forming a radical on most substituted carbon.
- Hydrogen from HBr attacks carbon radical.
What kind of regioselectivity is seen in free radical addition of HBr to an Alkene?
Explain the cause of the regioselectivity of free radical addition of HBr to an Alkene
The bromine radical is the species that attacks first in free radical addition, so it attaches to the carbon with the most hydrogens so that the most stable radical will be formed.
Why is free radical addition to an alkene only seen in HBr?
- The radicalization of HF is exothermic so the first step and the fourth step do not occur spontaneously.
- The H-Cl bond is strong enough to resist being broken by the carbocation in the fourth step so it does not occur spontaneously.
- Iodine has high radical stability so the attack by the pi electrons is exothermic and so the third step is not spontaneous.
What is a polymer?
A long chain of repeating units called monomers.
What is an addition polymerization reaction?
A reaction involving small monomer units added to a growing chain in very fast reactions.
Describe, in general, a free radical polymerization reaction.
- Monomer is an alkene.
- Homolytic pi bond cleavage by heat, putting a lone electron on each alkene carbon.
- Radical end of alkane attaches to a lone electron on polymer chain.
What is the polymer notation for a polymer made from a H2C=CH2 monomer?
What is the name of a polymer formed from ethylene monomers?
What are two ways to cleave the C=C bond in an alkene?
- Hot KMnO4
Describe the process of ozonolysis
- Ozone (O3) is pumped into an inert solvent containing an alkene.
- Ozone uses pi electrons to attach to alkene.
- A weak reducing agent (such as zinc or (CH3)2S) is added to reduce the oxygens.
- One of the oxygens leaves with the reducing agent and the other two oxygens form a C=O carbonyl group with the former alkene carbons.
What is different about the products of oxidation by hot KMnO4 and Ozonolysis?
In an oxidation reaction with hot KMnO4, in addition to oxidation of carbons forming carbonyl groups, all C-H bonds are oxidized to form C-O-H bonds.
What is the general mechanism for electrophilic addition and is it exothermic or endothermic?
- Pi bond attacks Y and Y-Z electrons collapse onto Z.
- Z attacks carbocation.
What is the regioselectivity of H-X electrophilic addition?
What is the intermediate in alkene halogenation?
bridged halonium ion
What is the intermediate of acid-catalyzed alkene hydration?
What is the regioselectivity of acid-catayyzed alkene hydration?
What are the conditions of acid-catalyzed hydration of an alkene?
What are the conditions for each of the two (2) steps of Oxymercuration-reduction?
- Hg(Oac)2, H20
What are the conditions for each of the two (2) steps of hydroboration-oxidation?
2. NaOH, H202
What are the conditions for a syn glycol formation (2 possibilities)?
- KMnO4 (cold, dilute), H20
2. OsO4, NaHSO3
What are the conditions for an anti glycol formation (2 steps)?
- Peroxyacid RC=OO2H
2. H+, H20
What are the conditions for the two (2) steps of ozonolysis?
2. (CH3)2S (or any weak reducing agent such as H20 and Zinc)
What is the more stable stereochemistry for an alkene? (2 components to answer)
(1) Trans because (2) steric interactions are reduced when large substituted groups are on opposite side of double bond.
What is the condition for a geometric isomer to exist?
Doubly bonded carbon atoms must be attached to two different atoms or groups by single covalent bonds.
What are four (4) preparation methods for alkenes?
- Dehydrohalogenation of alkyl halides
- Acid-catalyzed dehydration of alcohols
- Cis reduction of alkynes
- Trans reduction of alkynes
What is a beta-elimination reaction? (two components to answer)
A competing process to substitution where instead of a nucleophile attacking a carbon and removing a leaving group (substitution), a nucleophile (e.g. a strong base) attacks a hydrogen and causes the hydrogen’s electrons to collapse onto the carbon.
What is Zaitsev’s rule?
The major product of a beta-elimination reaction is the alkene with the greatest number of substituents on the carbon-carbon double bond.
What is an example of a reaction containing beta-elimination?
Dehydration of alcohols
What occurs in the concerted step of the E2 Dehydrohalogenation mechanism?
- (1) C-X electrons collapse on X and leaves as an anion; Strong base nucleophile attacks hydrogen attached to an adjacent carbon and C-H electrons collapse to form a double bond.
Describe the stereoselectivity of E2 dehydrohalogenation of an alkene (2 components to answer)
(1) The lowest-energy transition state of an E2 reaction is in which the leaving group and the beta hydrogen are anti coplanar (2) because this allows for proper orbital overlap between the base, the proton being removed and the departing leaving group.
Describe the regioselectivity of dehydrohalogenation of an alkene (which hydrogen is removed?)
USUALLY Zaitsev’s rule is followed so that the alkene formed is the most substituted and therefore most stable.
When is Zaitsev’s rule NOT followed in E2 elimination and what happens instead?
(1) When a bulky base such as tert-butoxide is used (2) hydrogen is removed from the least sterically hindered carbon.
Describe the conditions to dehydrate an alcohol into an alkene (2 conditions)
2. Strong acid
What are the three (3) steps of the mechanism for acid-catalyzed dehydration of an alcohol?
- Proton transfer from H30 to -OH of alcohol gives an oxonium ion
- C-O bond is broken and electrons collapse onto Oxygen forming a carbocation intermediate
- Nucleophile (water) attacks beta proton (hydrogen on carbon adjacent to carbocation) and C-H electrons collapse onto carbon and form the pi bond.
What is the reaction intermediate for acid-catalyzed dehydration of an alcohol?
How do you control the equilibrium between acid-catalyzed dehydration of an alcohol and hydration of an alkene? (2 components to answer)
- Large amounts of water (dilute acid) favors alcohol formation
- Scarcity of water (concentrated acid) favors alkene formation
Describe the regioselectivity of dehydration of an alcohol
What is catalytic reduction of an alkyne? (2 components)
(1) Hydrogen gas with a transition metal catalyst (2) will reduce an alkyne to an alkane.
How do you control the catalytic reduction of an alkyne to stop at alkene formation?
Use of Lindlar catalyst will result in cis alkene formation.
How do you reduce an alkyne to form a trans alkene?
What are the conditions of dissolving-metal reduction?
Sodium or Lithium dissolved in liquid ammonia (NH3)