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Topic 2
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The H atom in the
O-H bond can hydrogen bond with other alcohol molecules and with water, which
is why alcohols have relatively high boiling points and many are soluble in water
...
In pure
form it is used as a fuel and as a solvent, and in impure form is present in
alcoholic drinks
...
The manufacture of ethanol
Ethanol can be manufactured industrially in two ways – fermentation of sugars
and hydration of ethene
...

i)

fermentation of sugars

At 35 – 55 oC, sugars such as glucose can be fermented by yeast and turned into
ethanol and carbon dioxide
...
Some ethanol
made in this way is also used as fuels in countries such as Brazil, which have an
abundant supply of sugar cane
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2
...
A fuel derived or produced from renewable biological sources is
known as a biofuel
...
Although they release carbon dioxide when they are
burned, they come from plant sources which absorb carbon dioxide from the
atmosphere during photosynthesis while they are growing
...

3
...

Primary alcohols are those in which the carbon attached to the OH is attached to 0 or
1 other carbon atom
...

Eg propan-1-ol
H

H

C

C

C

H

H

H

H

H

OH

Secondary alcohols are those in which the carbon attached to the OH is attached to 2
other carbon atoms
...

Eg propan-2-ol

H

OH H

C

C

C

H

H

H

H

H

Tertiary alcohols are those in which the carbon atom attached to the OH is attached to
3 other carbon atoms
...

Eg 2-methypropan-2-ol
H

H

OH H

C

C

C

H

C

H

H

H

H

H

3
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Thus alcohols
tend to undergo nucleophilic substitution reactions
...
Thus
alcohols can also undergo elimination reactions
...

a)

elimination reactions

Like halogenoalkanes, alcohols can undergo elimination to give alkenes
...

The ethanol should be heated and passed over a catalyst (pumice can be used)
...

Alkenes produced in this way can be polymerised
...

The dehydration of alcohols is favoured by acidic conditions, as the -OH group
becomes protonated by H+ ions which produces a water molecule which then leaves
...
The detailed mechanism is not required
...
In some cases, this can lead to more than one product
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...
As the full equations are quite complex, the oxidising agent is
represented by the symbol [O]
...

Aldehydes and ketones are collectively known as carbonyls and can be represented
by the general formula CnH2nO
O
R

C
R

In aldehydes, one of the R groups is a H atom
...

b)

further oxidation of aldehydes

If an aldehyde is mixed with an oxidising agent, an oxygen atom can be added to the
group and a carboxylic acid will be formed:
Eg CH3CHO + [O]  CH3COOH
O
CH3

C

O

+

[O]

CH3

H

Ethanal

C
OH

Ethanoic acid

A carboxylic acid is a molecule containing the following group:
O
C
OH

Ketones cannot be oxidised into carboxylic acids since there is no C-H bond into
which an oxygen atom can be inserted
...

Cr2O72-(aq) + 14H+(aq) + 6e  2Cr3+(aq) + 7H2O(l)
The Cr2O72-(aq) ion is orange and the Cr3+ ion is green
...

If primary alcohols are oxidised, it is possible to form both aldehydes and carboxylic
acids
...

Carbonyls are more volatile than alcohols and carboxylic acids, since there is no
hydrogen bonding between aldehyde molecules
...
If reflux apparatus is

used, the aldehyde remains in the reaction vessel and is converted into the carboxylic
acid
...
Heat and an excess of the oxidising agent
also improve the yield of carboxylic acid
...
The distillation apparatus is
still favoured since the ketone is volatile so can be distilled off as it is formed
...

R-CH2OH + 2[O]  R-COOH + H2O (primary alcohol  carboxylic acid)
Excess K2Cr2O7, H2SO4, heat, reflux
R-CHO + [O]  R-COOH (aldehyde  carboxylic acid)
Excess K2Cr2O7, H2SO4, heat, reflux
R1-CH(OH)-R2 + [O]  R1-CO-R2 + H2O (secondary alcohol  ketone)
K2Cr2O7, H2SO4, heat, distillation

Summary of oxidation reactions of alcohols and carbonyls

primary
alcohols

secondary
alcohols

tertiary
alcohols
X

[O]

aldehydes

[O]

carboxylic
acids

[O]

ketones

4
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Aledehydes are reducing agents since they can be
oxidised to carboxylic acids, but ketones are not reducing agents
...

The presence of a “silver mirror” indicates that an aldehyde is present
...
Fehling’s solution is a complex solution containing Cu2+ ions
...
Cu2+ is an oxidising agent and will react with
aldehydes on heating
...

The presence of a brick red precipitate of Cu2O indicates that an aldehyde is
present
...
Summary of reactions of alcohols and carbonyls
Primary alcohol  aldehyde
Reagent: potassium dichromate and dilute sulphuric acid
Conditions: warm, distillation
Equation: RCH2OH + [O]  RCHO + H2O
Type of reaction: oxidation
Secondary alcohol  ketone
Reagent: potassium dichromate and dilute sulphuric acid
Conditions: heat, distillation
Equation: R1CH(OH)R2 + [O]  R1COR2 + H2O
Type of reaction: oxidation
aldehyde  carboxylic acid
Reagent: potassium dichromate and dilute sulphuric acid
Conditions: heat, reflux
Equation: R-CHO + [O]  R-COOH
Type of reaction: oxidation
Alcohols  alkenes
Reagent: concentrated sulphuric acid
Conditions: heat
Equation: R1R2CHC(OH)R3R4  R1R2C=CR3R4 + H2O
Type of reaction: elimination



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