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A Level Chemistry > 6.1 Organic chem and analysis 25.1-26.4 > Flashcards

6.1 Organic chem and analysis 25.1-26.4 Flashcards

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1
Q

benzene

A

hexagonal ring of 6 carbon atoms, with each carbon atom joined to two other carbon atoms and to one hydrogen atom
classified as an aromatic hydrocarbon or arene
sweet smelling
C6H6

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2
Q

the kekule model of benzene

A

six membered ring of carbon atoms joined by alternate single and double bonds

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3
Q

evidence to disprove kekule’s model of benzene

A
  1. lack of reactivity of benzene- if benzene contained C=C bonds then it should decolourise bromine in an electrophilic addition reaction. however it doesn’t undergo electrophilic addition and doesn’t decolourise bromine under normal conditions
  2. lengths of carbon-carbon bonds in benzene- all bond lengths the same. should be different for double and single bonds.
  3. hydrogenation enthalpies- the hydrogenation enthalpy of benzene is expected to be three times that of cyclohexene, if it were Kekulé’s model. However, it is smaller, and less energy is produced than expected, meaning that the actual model of benzene is a lot more stable than Kekulé’s model.
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4
Q

delocalised model of benzene

A
  • benzene is a planar, cyclic, hexagonal hydrocarbon containing six carbon atoms and six hydrogen atoms
  • each carbon atom uses three of its available four electrons in bonding to two other carbon atoms and to one hydrogen atom
  • each carbon atom has one electron in a p-orbital at right angles to the plane of the bonded carbon and hydrogen atoms
  • adjacent p-orbital electrons overlap sideways, in both directions, above and below the plane of the carbon atoms to form a ring of electron density.
  • this overlapping of the p-orbitals creates a system of pi-bonds which spread over all six of the carbon atoms in the ring structure
  • the six electrons occupying this system of pi-bonds are said to be delocalised
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5
Q

naming aromatic compounds with one substituent group

A

monosubstituted
the benzene ring is often considered to be the parent chain
alkyl groups (CH3, C2H5), halogens (F,Cl,Br,I) and nitro (NO2) groups are all considered the prefixes to benzene
when a benzene ring is attached to an alkyl chain with a functional group or to an alkyl chain with seven or more carbon atoms, benzene is considered to be a substituent. instead of benzene, the prefix phenyl is used in the name.

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6
Q

phenyl

A

a cyclic group of atoms with the formula C6H5. Phenyl groups are closely related to benzene and can be viewed as a benzene ring, minus a hydrogen, which may be replaced by some other element or compound to serve as a functional group.

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7
Q

benzoic acid

A

C6H5COOH

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8
Q

phenylamine

A

C6H5NH2

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9
Q

benzaldehyde

A

C6H5CHO

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10
Q

naming aromatic compounds with more than one substituent group

A

the ring is numbered like a carbon chain, starting with one of the substituent groups
the substituent groups are listed in alphabetical order using the smallest numbers possible

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11
Q

reactivity of benzene and its derivatives

A

they undergo substitution reactions in which a hydrogen atom on the benzene ring is replaced by another atom or group of atoms
benzene typically reacts with electrophiles and most of the reactions of benzene proceed by electrophilic substitution

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12
Q

nitration of benzene

A

electrophilic substitution
benzene reacts slowly with nitric acid to form nitrobenzene
the reaction is catalysed by sulfuric acid and heated to 50 degrees C to obtain a good rate of reaction
in nitration, one of the hydrogen atoms on the benzene ring is replaced by a nitro, -NO2, group

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13
Q

nitration of benzene mechanism step 1

A

the reaction involves an electrophile, however nitric acid is not the electrophile involved in the mechanism
the elctrophile in this reaction is the nitronium ion, NO2+, produced by the reaction of concentrated nitric acid with concentrated sulfuric acid
HNO3 + H2SO4 –> NO2+ + HSO4- + H2O

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14
Q

nitration of benzene mechanism step 2

A

the electrophile, NO2+, accepts a pair of electrons from the benzene ring to form a dative covalent bond
the organic intermediate formed is unstable and breaks down to form the organic product, nitrobenzene, and the H+ ion. A stable benzene ring is reformed

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15
Q

nitration of benzene mechanism step 3

A

the H+ ion formed in step 2 reacts with the HSO4- ion from step 1 to regenerate the catalyst H2SO4
H+ + HSO4- –> H2SO4

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16
Q

halogenation of benzene

A

the halogens dont react with benzene unless a catalyst called a halogen carrier is present
common halogen carriers include AlCl3. FeCl3, AlBr3 and FeBr3

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17
Q

bromination of benzene

A

at room temp and pressure and in the presence of a halogen carrier, benzene reacts with bromine in an electrophilic substitution reaction
in bromination, one of the hydrogen atoms on the benzene ring is replaced by a bromine atom

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18
Q

bromination of benzene mechanism step 1

A

benzene is too stable to react with a non polar bromine molecule
the electrophile is the bromonium ion, Br+, which is generated when the halogen carrier catalyst (e.g. FeBr3) reacts with bromine
Br2 + FeBR3 –> FeBr4- + Br+

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19
Q

bromination of benzene mechanism step 2

A

the bromonium ion accepts a pair of electrons from the benzene ring to form a dative covalent bond
the organic intermediate is unstable and breaks down to form the organic product, bromobenzene and an H+ ion

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20
Q

bromination of benzene mechanism step 3

A

the H+ formed in step 2 reacts with the FeBr4- ion from step 1 to regenerate the FeBr3 catalyst
H+ + FeBr4- –> FeBr3 + HBr

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21
Q

chlorination of benzene

A

chlorine will react with benzene in the same way as bromine and following the same mechanism

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22
Q

alkylation of benzene

A

substitution of a hydrogen atom in the benzene ring by an alkyl group
the reaction is carried out by reacting benzene with a haloalkane in the presence of AlCl3, which acts as a halogen carrier catalyst, generating the electrophile
alkylation increases the number of carbon atoms in a compound by forming carbon-carbon bonds
also known as friedal-crafts alkylation

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23
Q

acylation reactions with benzene

A

The acylation of benzene adds an acyl group, (RCO-) to the ring.
The reaction involves the transformation of the acyl chloride into an electrophile using an AlCl3 catalyst.
RCOCl + AlCl3 → RCO+ + AlCl4-
This electrophile can then undergo the usual electrophilic substitution
Firstly, the attack of the electrophile on the electron-rich benzene ring to form the positive, non-aromatic intermediate.
Secondly, the proton is removed. This is done using the AlCl4− to react with this proton to form: HCl + AlCl3.

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24
Q

compare the reactivity of alkenes with arenes

A

alkenes decolourise bromine by an electrophilic addition reaction.
benzene doesnt react with bromine unless a halogen carrier catalyst is present. this is beacuse benzene has delocalised pi-electrons spread above and below the plane of the carbon atoms in the ring structure. the electron density around any two carbon atoms in the benzene ring is less than that in a C=C double bond in an alkene

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25
phenols
organic chemical containing a hydroxyl, -OH, functional group directly bonded to an aromatic ring. the simplest member of the pheols, C6H5OH, has the same name as the group- phenol.
26
phenols and water
phenol is less soluble in water than alcohols due to the presence of the non polar benzene ring when dissolved in water, phenol partially dissociates forming the phenoxide ion and a proton
27
why is phenol classified as a weak acid?
because of its ability to partially dissociate in water to produce protons
28
reaction of phenol with sodium hydroxide
forms the salt, sodium phenoxide, and water in a neutralisation reaction
29
bromination of phenol
electrophilic substitution reaction phenol reacts with an aqueous solution of bromine to form a white ppt of 2,4,6-tribromophenol the reaction decolourises the bromine water a halogen carrier catalyst is NOT required and the reaction is carried out at room temp
30
nitration of phenol
electrophilic substitution reaction reacts readily with dilute nitric acid at room temp a mix of 2-nitrophenol and 4-nitrophenol is formed
31
comparison of the reactivity of phenol and benzene
bromine and nitric acid react more readily with phenol than benzene. phenol is nitrated with dilute HNO3 rather than needing conc nitric and sulfuric acids as with benzene phenol more reactive
32
why is phenol more reactive than benzene
phenol's inc reactivity is caused by a lone pair of electrons from the oxygen p-orbital of the -OH group being donated to the pi-system of phenol. the electron density of the benzene ring in phenol is increased. the increased electron density attracts electrophiles more strongly than with benzene
33
directing effects
there are many diff groups that can be attached to a benzene ring. diff groups can have a directing effect on any second substituent on the benzene ring all 2- and 4-directing groups are activating groups , besides the halogens all 3-directing groups are deactivating groups
34
2- and 4-directing groups
ortho-and-para directing -NH2 -OH activating groups
35
3-directing groups
meta directing -NO2 deactivating group
36
carbonyl functional group
C=O | aldehydes and ketones
37
aldehydes vs ketones
aldehydes- C=O group at end of a carbon chain. CHO | ketones- C=O is joined to two carbon atoms in the carbon chain. CO
38
oxidation of aldehydes
oxidised to carboxylic acids when refluxed with acidified dichromate (VI) ions, usually a mix of sodium or potassium dichromate(VI), K2Cr2O7, and dilute sulfuric acid , H2SO4
39
oxidation of ketones
dont undergo oxidation reactions | lack of reactivity
40
how can chemists distinguish between aldehydes and ketones?
ketones dont undergo oxidation reactions, not very reactive
41
reaction of carbonyl compounds with NaBH4
sodium tetrahydridoborate is used as a reducing agent to reduce aldehydes and ketones to alcohols the aldehyde or ketone is usually warmed with the NaBH4 reducing agent in aqueous solution nucleophilic addition
42
reducing an aldehyde
reduced to primary alcohols by NaBH4 | nucleophilic addition
43
reducing a ketone
reduced to secondary alcohols by NaBH4 | nucleophilic addition
44
reaction of carbonyl compound with HCN
hydrogen cyanide adds across the C=O bond of aldehydes and ketones sodium cyanide and sulfuric acid are used to provide the hydrogen cyanide in the reaction, H2SO4 and NaCN forms hydroxynitriles -OH and C triple bond N groups
45
mechanism for carbonyl compounds reaction with NaBH4
nucleophilic addition. NaBH4 contains hydride ion H-, acts as nucleophile 1. lone pair of elecs from hydride ion is attracted and donated to the delta+ carbon atom in the aldehyde/ketone C=O bond 2. a dative covalent bond is formed between the hydride ion and the carbon atom of the C=O bond 3. the pi-bond in the C=O bond breaks by heterolytic fission, forming a negatively charged intermediate 4. the oxygen atom of the intermediate donates a lone pair of electrons to a hydrogen atom in a water molecule. the intermidiate has then been protonated to form an alcohol
46
mechanism for carbonyl compounds reaction with NaCN/H+
cyanide ion CN- attacks the elec deficient carbon atom in the aldehyde/ketone 1. lone pair of elecs from the cyanide ion is attracted and donated to the delta+ carbon atom in the aldehyde/ketone C=O bond 2. the pi-bond in the C=O bond breaks by heterolytic fission, forming a negatively charged intermediate 3. the intermediate is protonated by donating a lone pair of elecs to a hydrogen ion, to form the product 4. the product is hydroxynitrile
47
testing for carbonyl compounds
2,4-DNP used | in the presence of a carbonyl group, a yellow or orange ppt is produced
48
distinguishing between aldehydes and ketones after testing for carbonyl group
use tollens reagent - a solution of silver nitrate in aqueous ammonia in the presence of an aldehyde group, a silver mirror is produced contains silver ions, Ag+, which act as an oxidising agent in the presence of ammonia. in the reaction, silver ions are reduced to silver as the aldehyde is oxidised to a carboxylic acid
49
tollens reagent
a solution of silver nitrate in aqueous ammonia | in the presence of an aldehyde group, a silver mirror is produced
50
identifying an aldehyde or ketone by MP
the ppt formed in the 2,4-DNP test can be analysed 1. the impure orange solid is filtered to separate the solid ppt from the solution 2. solid is then recrystallised to produce a pure sample of crystals 3. the MP of the pure crystals is measured and recorded the MP is compared to a database of MPs to identify the original carbonyl compound
51
the carboxyl group
contains a carbonyl groups and a hydroxyl group
52
solubility of carboxylic acids
the C=O and O-H bonds are polar allowing carboxylic acids to form hydrogen bonds with water molecules - carboxylic acids with up to 4 C atoms are soluble in water - as the no of C atoms increases, the solubility decreases as the non-polar carbon chain has a greater effect on the overall polarity of the molecule
53
strength of carboxylic acids
weak acids | when dissolved in water, carboxylic acids partially dissociate
54
acid reactions of carboxylic acids
carboxylic acids take place in redox reactions with metals and neutralisation reactions with bases (alkalis, metal oxides and carbonates) in these reactions carboxylic acids form carboxylate salts
55
redox reactions of carboxylic acids with metals
aq solutions of carboxylic acids react with metals in a redox reaction to form hydrogen gas and the carboxylate salt you would observe the metal disappearing and effervescence as H gas is evolved.
56
neutralisation reactions of carboxylic acids with bases
carboxylic acids react with all bases- metal oxides, alkalis and carbonates.
57
neutralisation reactions of carboxylic acids with metal oxides
form salt and water | e.g. 2CH3COOH + CaO --> (CH3COO-)2Ca+2 + H2O
58
neutralisation reactions of carboxylic acids with alkalis
form a salt and water
59
neutralisation reactions of carboxylic acids with carbonates
forms salt, water and CO2
60
test for the carboxyl group
carboxylic acids are the only common organic compounds sufficiently acidic to react with carbonates neutralisation reaction with carbonates
61
what is a derivative of a carboxylic acid?
a compound that can be hydrolysed to form a parent carboxylic acid. carboxylic acid derivatives have a common sequence of atoms in their structure, known as an acyl group
62
what are the derivatives of carboxylic acids?
ester acyl chloride acid anhydride amide
63
esters
names after the parent carboxylic acid from which it is derived to name just remove -oic acid suffic from the parent carboxylic acid and replace with -oate the alkyl chain attached to the O atom of the COO group is then added as the first word in the name
64
acyl chlorides
names after the parent carboxylic acid from which it is derived to name remove the -oic acid suffix and replace with -oyl chloride
65
acid anhydrides
formed by removal of water from two carboxylic acid molecules
66
esterification
reaction of an alcohol with a carboxylic acid to form an ester. an alcohol is warmed with a carboxylic acid with a small amount of conc sulfuric acid, which acts as a catalyst esters are sweet smelling liquids
67
hydrolysis of esters
esters can be hydrolysed by aq acid or alkali | hydrolysis is the chemical breakdown of a compound in the presence of water or in aq solution
68
acid hydrolysis of esters
reverse of esterification - ester is heated under reflux with dilute aq acid - the ester is broken down by water, with the acid acting as a catalyst the products are a carboxylic acid and an alcohol
69
alkaline hydrolysis of esters
irreversible ester is heated under reflux with aq hydroxide ions carboxylate ion and alcohol formed
70
preparation of acyl chlorides
can be prepared directly from their parent carboxylic acid by reaction with SOCl2 produces acyl chlorides, SO2 and HCL should be carried out in fume cupboard
71
reactions of acyl chloride
very reactive | react with nucleophiles by losing the chloride ion whilst remaining the C=O bond
72
reaction of acyl chlorides with alcohols
forms esters and HCl
73
reaction of acyl chlorides with phenols
forms esters and HCl | no catalyst needed. reactive enough on own
74
reaction of acyl chlorides with water
forms carboxylic acids and HCl | violent reaction takes place with the evolution of dense steamy HCl fumes
75
reaction of acyl chlorides with ammonia and amines
forms amides and ammonium chlorides ammonia and amides act as nucleophiles by donating lone pair of elecs on the N atom to an elec deficient species -ammonia reacts with acyl chloride, forming a primary amide -a primary amine reacts with an acyl chloride in the same way to form a secondary amide
76
reactions of acid anhydrides
react in similar way to acyl chlorides with alcohols, phenols, water, ammonia and amines less reactive react w phenol to form ester and carboxylic acid

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