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ALCOHOLS


Alcohols contain the -OH functional group.
General formula – CnH2n+1OH
Naming Alcohols

The first part of the name of an alcohol is according to the longest carbon atom sequence. The second part of the name is –ol. A
number will be included to indicate the position of the alcohol group.

If the compound has an –OH group in addition to other functional group that need a suffix ending then the –OH can be named with
the prefix

hydroxy):
If there are two or more –OH groups then di, tri are used. Add the `e` on to the stem name though.
Bond Angles In Alcohols
All the H-C-H bonds and C-C-O
are 109.50 (tetrahedral shape),
because there are 4 bond pairs of
electrons repelling to a position of
minimum repulsion.
The H-O-C bond is 104.50 (bent line
shape), because there are 2 bond pairs of
electrons and 2 lone pairs repelling to a
position of minimum repulsion. Lone
pairs repel more than bond pairs so the
bond angle is reduced.
Types Of Alcohols
There are three types of alcohol; primary, secondary and tertiary.
They are classified according to the number of carbon groups attached to the carbon with the OH group.
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Physical properties of alcohols

The OH group can take part in hydrogen-bonding, both as a donor (H is sufficiently +) and as an acceptor (through the two lone pairs on
the oxygen atom). As a result alcohols have higher melting and boiling points than hydrocarbons of comparable molar mass.

They are also more soluble in water because of H-bonding: ethanol is miscible with water in all proportions. As the non-polar
hydrocarbon chain becomes longer, it becomes harder for water to dissolve the alcohol: from C4 alcohols are less soluble, and don’t mix
with water.
Reactions of Alcohols
1. Combustion
All alcohols undergo combustion to form carbon dioxide and water. For example the equation for the combustion of butanol is as follows;
C4H9OH + 6O2  4CO2 + 5H2O
Alcohols combust with a clean flame
2. Reaction With Sodium
All alcohols react with sodium.
2ROH + 2Na  2RONa + H2
This equation is similar to the reaction of sodium with acid. The salt formed here is an alkoxide.
e.g. Ethanol and sodium
2C2H5OH + 2Na  2 C2H5ONa + H2
Sodium ethoxide
Note: This reaction is used to test for alcohols.
Observations

Effervescence

The mixture gets hot

Sodium dissolves

A white solid is formed
3. Nucleophilic Substitution Reactions of Alcohols To Form Halogenoalkanes
Various halogenating compounds can be used to substitute the –OH group for a
halogen
a) Reaction with phosphorous (V) halide
ROH + PCl5  RCl + POCl3 + HCl
e.g. Propanol and phosphorus pentachloride
CH3CH2CH2OH + PCl5  CH3CH2CH2Cl + POCl3 + HCl
Note: This reaction with PCl5 (phosphorous(v)chloride) can be used as a test for alcohols. You would observe white misty fumes of HCl
produced.
NOTE:
PCl5 / PCl3 / conc HCl / SOCl2 / mixture of NaCl + H2SO4 can all be uses
for substituting a Cl
So,



3CH3CH2CH2OH + PCl3  3CH3CH2CH2Cl + H3PO4
CH3CH2CH2OH + SOCl2  CH3CH2CH2Cl + SO2 +
C2H5OH (l) + HCl (g)  C2H5Cl (l) + H2O (l)
HCl
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b)
Forming Bromo and Iodoalalkanes
NOTE:
For Br and I it is best to use PI5 , PI3 and Br equivalents. It is
not suitable to use NaBr, or NaI + H2SO4 to produce HBr or
HI because the sulphuric acid with oxidise the hydrogen
halides to other products
i.e.
C2H5OH + HBr  C2H5Br + H2O
PI3 + 3 CH3CH2OH  3CH3CH2 I + H3PO3
(The phosphorous (III) halide can be produced in situ by
reacting red phosphorus and the halogen).
The relative reactivity’s of alcohols in halogenation are tertiary > secondary > primary alcohol.
4. Oxidation Of Alcohols

Alcohols can be oxidised with a mixture of dilute sulphuric acid with sodium or potassium dichromate(VII) solution,
which together act as oxidising agents.

The exact reaction, however, depends on the type of alcohol, i.e. whether it is primary, secondary, or tertiary, and on the
conditions.


In writing equations for these oxidation reactions [O] is used to represent the oxidising agent.
When alcohols do react with acidified potassium or sodium dichromate VI a colour change from orange to green is
seen.
1 Primary alcohols form an aldehyde, and then on further oxidation, form carboxylic acids.
0-
Cr2O72-/H+
Heat/reflux
Cr2O72-/H+
Heat/disti
l
20 - Secondary alcohols form ketones, but no further oxidation takes place.
Cr2O72-/H+
Heat/reflux
30 - Tertiary alcohols do not react with oxidizing agents.
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Equations Showing Partial oxidation of 10 Alcohols
Observation:
the orange dichromate ion (Cr2O72-) reduces
to the green Cr 3+ ion
Note:
An aldehyde’s name ends in –al It always has
the C=O bond on the first carbon of the chain so
it does not need an extra number
Equations Showing The full Oxidation Of 10 Alcohols
Observation:
the orange dichromate ion (Cr2O72-)
reduces to the green Cr3+ ion
Equations Showing The Oxidation of 20 Alcohols
When ketones have 5C’s or more in a chain
then it needs a number to show the position
of the double bond. E.g. pentan-2-one
Observation:
The orange dichromate ion (Cr2O72-)
reduces to the green Cr3+ ion
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Partial Oxidation Of Alcohols To Aldehydes (Distillation)
e.g. ethanol to ethanal.
CH3CH2OH + [O]  CH3CHO + H2O
To make the aldehyde, the one reagent is added dropwise to
the other and the product is distilled off as it forms.
By distilling the aldehyde off as it forms, it means it will not
undergo further oxidation to the acid.
Note

the bulb of the thermometer should be at the T
junction connecting to the condenser to
measure the correct boiling point

the water goes in the bottom of the condenser
to go against gravity. This allows more efficient
cooling and prevents back flow of water.
Complete Oxidation Of Alcohols To Carboxylic Acids
e.g. ethanol to ethanoic acid. The mixture of reagents is heated under reflux.
CH3CH2OH + 2[O]  CH3CO2H + H2O
Heating under reflux
The apparatus shown here is used a large number of organic preparations.
The reaction mixture is placed in the pear shaped flask. It has a reflux condenser*
fitted. This means that as the reactants are heated and the volatile liquids boil off, they
are converted back to liquid in the condenser and return to the flask.
*A reflux condenser “is not a special type of condenser, it is an ordinary condenser fitted
so that reflux takes place.
NOTE:

Reflux is used when heating organic reaction mixtures
for long periods. The condenser prevents organic
vapours from escaping by condensing them back to
liquids.

Never seal the end of the condenser as the build up of
gas pressure could cause the apparatus to explode. This
is true of any apparatus where volatile liquids are
heated including the distillation set up

Anti-bumping granules are added to the flask in both
distillation and reflux to prevent vigorous, uneven
boiling.
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Once the carboxylic has been formed, it needs to be separated from the reaction mixture and other products. This is done by distillation.
i.e.
Distillation is used to separate a volatile product
from a mixture of involatile substances, or
substances that have a boiling point of at least 50oC
higher than the component being collected
Distinguishing between Aldehydes and Ketones
The fact that aldehydes can be further oxidised to carboxylic acids whereas ketones cannot be further oxidised is the chemical basis for tests
that are commonly used to distinguish between aldehydes and ketones
Fehling’s (Benedict’s) solution
NOTE:
2+
Reagent: Fehling’s Solution containing blue Cu ions.
Conditions: heat gently
Reaction: aldehydes only are oxidised by Fehling’s solution into a
carboxylic acid and the copper ions are reduced to copper(I) oxide
Observation:
Aldehydes : Blue Cu2+ ions in solution change
to a red precipitate of Cu2O. Ketones do not react Fehling’s
(Benedict’s) solution
CH3CHO + 2Cu2+ + 2H2O  CH3COOH + Cu2O + 4H+
The presence of a carboxylic acid can be tested
by addition of sodium carbonate. It will fizz
and produce carbon dioxide
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Purifying an organic liquid

Put the distillate of impure product into a separating funnel. Wash product by adding either:1.
sodium hydrogencarbonate solution , shaking and releasing the pressure from CO2
produced. Sodium hydrogencarbonate will neutralise any remaining reactant acid.
2.
Saturated sodium chloride solution. Sodium chloride will help separate the organic layer
from the aqueous layer
Allow the layers to separate in the funnel, and then run and discard the aqueous layer.
Run the organic layer into a clean, dry conical flask and add three spatula loads of drying agent
(anhydrous sodium sulphate) to dry the organic liquid.
The drying agent should:
a)
be insoluble in the organic liquid
b)
not react with the organic liquid
• Carefully decant the liquid into the distillation flask
•Distill to collect pure product
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