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Chirality, Carbonyls and Carboxylic Acids
Questions on this unit may include material from UNIT 2 – see syllabus
Isomerism
Structural isomerism
...

All isomers are compounds with the same molecular formula
...
g
...

Structural isomers have atoms arranged in different orders
...

e
...


CH 3 CH 2 CH 2 CH 3 (butane) and CH 3 CH(CH 3 )CH 3 (2-methylpropane)
CH 3 CH 2 OH (ethanol) and CH 3 OCH 3 (methoxymethane)

Stereoisomerism
...

The same atoms are arranged in the same order but with different orientations in space
...

Another form is optical isomerism
...
Optical isomerism occurs when two compounds have the
same molecular formula, but are not superimposable on each other
...

Example CH 3 CH(OH)CO 2 H

H

H

C

C

CH 3 CH(OH)CO 2 H

HO
H3C

CO2H

HO2C

OH
CH3

The two isomers affect polarised light by rotating the plane of polarisation of plane polarised
monochromatic light in opposite directions; this is where the term "optical" comes from
...
Optical isomers exist in two forms called enantiomers
...

A sample of an optically active substance may contain both optically active isomers
...
This
optically inactive mixture is called the racemic mixture or racemate
...

The two mirror image molecules are said to be chiral
...

When this group occurs at the end of a carbon chain, the compound is an aldehyde (RCHO), the
name ending in –al
...


H
H C
H
H
H C
H

H
C
H

H
C
H
Pentanal
H
C
H

O

H
C
H

H
C

C
H

O
H
H
C
C
C
H
H
Pentan-3-one

H
H C
H

O
C
H

O
C

H
C

H
C H
H
H
C H
H

H C H
3-Methylbutan-2-one H

H C H
2-Methylbutanal

H
H C
H

H

The carbonyl group in polar because of the electronegative oxygen atom
...


Physical Properties
Carbonyl compounds are much more volatile than the corresponding alcohol because, unlike
alcohols, they do not have any hydrogen bonding
...

Compound
Formula
Formula mass
Boiling point /oC
Propane
C3H8
44
-42
Ethanal
CH 3 CHO
44
20
Ethanol
C 2 H 5 OH
46
78
Although carbonyl compounds do not have a hydrogen which is directly connected to the oxygen,
and therefore they have no hydrogen bonding, when placed in water the oxygen in the carbonyl is
able to form a hydrogen bond with the hydrogen in the water molecules
...


Chemical Properties
Aldehydes and ketones are both attacked by nucleophiles, and can both be reduced to alcohols
...

Aldehydes and ketones are obtained by oxidation of primary and secondary alcohols,
respectively
...

All carbonyl compounds react with 2,4-dinitrophenylhydrazine (Brady's reagent)
...

This is used as the test for the presence of the carbonyl group
...

Since ketone cannot be oxidised, they do not take part in oxidation reactions
...

Aldehydes will react when heated with ammoniacal silver nitrate solution (Tollen's reagent)
...

When carried out in a clean test tube it forms a silver mirror
...
These tests are summarized below;
Oxidising agent

Tollen’s reagent
Fehling’s solution
Dichromate solution

Conditions
Heat aldehyde with ammoniacal
silver nitrate
Heat aldehyde with Fehling’s
solution
Heat aldehyde with a mixture of
potassium dichromate solution and
sulphuric acid

Result for aldehyde
Silver mirror forms
Turns from a blue solution to
form a red precipitate
Turns from an orange solution
to a green solution

Aldehydes with Fehling’s solution (oxidation)
Aldehydes (but not ketones) reduce Cu2+ to Cu+ giving a red brown precipitate of copper (I) oxide
in this test
...

The mixture is warmed in a water bath
...

The reverse of this process, reduction, converts carbonyls back into alcohols[*]
...

Note - In equations showing reduction the reducing agent is written as [H]
...
g
...


CH 3 CH 2 CHO + 2[H] 

CH 3 CH 2 CH 2 OH



CH 3 CH(OH)CH 3

The reduction of propanone to propan-2-ol
...

In this case the attack is by a nucleophile being drawn to the molecule by the partial positive
charge on the carbon
...

To carry out this reaction, a mixture of potassium cyanide and ethanoic acid is used to avoid
use of the very poisonous hydrogen cyanide
...

Ethanal and HCN

O
C
H3C

OH
+ KCN, pH 8
(HCN)
H

-4–

CH
H3C

CN

Propanone and HCN

O
C
H3C

OH
+ KCN, pH 8
(HCN)
CH

C
H3C

3

CN
CH3

Reaction mechanism
In this reaction the initial attack is by the cyanide ion
...

The reaction has to be carried out in slightly basic conditions, so the mixture is buffered at pH8
...

H+ + CN-

HCN

If the pH is lower than 8, the dissociation of the HCN lies too far to the left and so the
concentration of CN- is too low for the first step to take place
...


C


...



...


Reaction of iodine in alkali
Iodine in alkali reacts with a specific group, CH 3 COR to produce a yellow precipitate of CI 3 H
...

This is used as a test for the presence of the CH 3 COR group
...

The iodine present in the testing reagent can oxidise alcohols, and so CH 3 CHOHR will initially
form CH 3 COR this will then react to give the yellow crystals
...

The test is carried out by adding aqueous sodium hydroxide to iodine solution until the mixture
just turns colourless
...

CH 3 COR and CH 3 CHOHR give a positive iodoform reaction (where R can be a carbon chain or
hydrogen)
...

The name carboxyl comes from a combination of the names of these functional groups;
Carbonyl + hydroxyl = Carboxyl

H
H C
H

O

H
C
H

H
H C
H

C
H

O

H
C
H

H
H C
H

O

H
C

C
O

O

H
C
H

C
H

O

Butanoic acid

Propanoic acid

H
H C
H

H
C
H

H

H
C

H
C
H

O
C
O

H

H C H

H C H

3-Methylbutanoic acid

H

2-Methylbutanoic acid H

Physical Properties
The Carboxyl carbon contains two oxygen atoms both of which are electronegative leaving the
carbon with a partial positive charge
...

Compound
Propanol
Ethanoic acid

Formula
CH 3 CH 2 CH 2 OH
CH 3 CO 2 H

RFM
60
60

Boiling point /oC
97
118

The structure of the carboxyl group allows carboxylic acid to form dimers

O
R

H

O

C

C
O

R

O

H

Ethanoic acid has a melting temperature of 17oC, so if the temperature falls below this it freezes,
and the similarity of frozen ethanoic acid to ice has given the pure acid the common name of
glacial ethanoic acid
...

The longer the carbon chain, the less soluble in water the carboxylic acid becomes
...

 RCO 2 H + H 2 O
RCH 2 OH + 2[O]
Preparation from aldehydes
When an aldehyde is heated under reflux with potassium dichromate and sulphuric acid a
carboxylic acid is produced
...

The hydrolysis can be carried out by heating the nitrile with acid or alkali
...


RCO 2 H + NH 4 Cl
RCN + 2H 2 O + HCl
Hydrolysis using aqueous sodium hydroxide produces the salt of the carboxylic acid
...

RCO 2 Na + HCl

RCO 2 H + NaCl

Chemical Properties
Reduction
Carboxylic acids are formed by the oxidation of primary alcohols, and can be converted back to
these compounds using lithium tetrahydridoaluminate(III) (lithium aluminium hydride} as a
reducing agent
...

e
...
Reduction of propanoic acid
...


HCO 2 H + 4[H]




CH 3 CH 2 CH 2 OH + H 2 O
CH 3 OH + H 2 O

Reaction with alcohols
Carboxylic acids react with alcohols in the presence of concentrated sulphuric acid to form water
and an ester
...

The mixture is then warmed
...
g
...

-7–

The esters formed contain the ester functional group or ester link
...

RCOOH + R*OH 
RCOOR* + H 2 O

O
C

R

O

R*

Ester group

Esters have characteristic odours, which makes them useful for flavouring
...
Esters are also useful as solvents
...
These reactions occur at room
temperature
...
This functional
group has the structure shown below
...


CH 3 COCl + HCl + POCl 3
CH 3 CO 2 H + PCl 5
Ethanoyl chloride
Propanoic acid reacts with phosphorus pentachloride to produce propanoyl chloride
...

Reaction with sodium hydroxide

RCO 2 H + NaOH

Reaction with sodium carbonate
...
For example this technique can be used to find
the quantity of citric acid in fruit
...

This includes acyl chlorides and esters
...

When hydrolysed by acid, the alcohol and the carboxylic acid are reformed
...

R1CO 2 H + R2OH
R1COOR2 + H 2 O
When hydrolysed by an alkali, the alcohol and the salt of the carboxylic acid are formed
...

Such a reaction is called saponification
...
g
...

An animal fat is an ester formed from propan-1,2,3-triol and carboxylic acids with long chains
...


-9–

The structure of the salt of the carboxylic acid is shown below
...
The charged section is attracted
to the polar water molecules – is hydrophilic
...
The soap molecule can be pictured as like a tadpole with
“hydrophilic head” and “hydrophobic tail”
...
This breaks down
the surface tension of the water and allows it to wet a
surface
...
Such a polymer is called a polyester
...

nHOCH 2 CH 2 OH + nHO 2 C-C 6 H 4 -CO 2 H
ethane-1,2-diol
benzene-1,4dicarboxylic acid



-(-CH 2 CH 2 -O-CO-C 6 H 4 -CO-O-)- n + nH 2 O
Terylene

Transesterification
The burning of diesel oil from petroleum is not an environmentally sustainable method of
providing energy
...

Such oils will only partially combust in a normal diesel engine, and will therefore cause clogging of
the engine
...

An alternative is to convert the triglyceride in the fat or oil to a methyl ester
...
The methyl ester can be formed in a
process called transesterification
...

They are readily hydrolysed at room temperature, and will fume in moist air due to this reaction
...

Reaction with water
RCOCl + H 2 O  RCO 2 H + HCl

Reaction with alcohols
They react with alcohols at room temperature to produce the ester
...

RCOCl + NH 3  RCONH 2 + HCl

Reaction with amines
They react with amines at room temperature to produce secondary substituted amides
...

RCOCl + R#NH 2  RCO-NH-R#

- 11 –



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