School of Advance Level Studies
Chemistry AS Notes 2023 Ethanol the biofuel
could it be the fuel
Organic Chemistry of the future?
Read
Hydroxy compounds: Alcohols
introduction to chapter 17
Alcohols are organic molecules containing at least one hydroxyl 3rd edition of the Chem
group, -OH. With one hydroxyl group substituted into an alkane to learn more.
molecule, the general formula is CnH2n + 1OH.
For alcohol molecules with three or more carbon atoms, the
position of the hydroxyl group is shown by inserting a number
to indicate which carbon atom is bonded to the OH group.
Classification of alcohols.
Alcohols are classified according to the number of carbon atoms attached to the carbon atom
bonded to the -OH group:
Primary alcohols are alcohols in which the carbon atom bonded to the -OH group is
attached to one other carbon atom (or alkyl group)
Secondary alcohols are alcohols in which the carbon atom bonded to the -OH group
is attached to two other carbon atoms (or alkyl groups)
Tertiary alcohols are alcohols in which the carbon atom bonded to the -OH group is
attached to three other carbon atoms (or alkyl groups)
Remember :
Characteristic distinguishing reactions:
Only primary and secondary alcohols can get oxidised when mildly oxidised with acidified
K2Cr2O7. Primary alcohols get mildly oxidised to aldehydes and secondary alcohols get
mildly oxidized to ketones. Tertiary alcohols do not undergo oxidation with acidified
K2Cr2O7.
Therefore, only the oxidation of primary and secondary alcohols will change the colour of
K2Cr2O7 solution as the orange Cr2O72- ions are reduced to green Cr3+ ions (figure 17.8).
1
, Acidity of alcohol
Alcohols have a low degree of
dissociation. This means, that when
dissolved in water, alcohol molecules
do not dissociate (split up) to a great
extent.
The position of the equilibrium lies to
the left, meaning that there are far more
alcohol molecules than RO– and
H+ ions.
When water dissociates, the position of
the equilibrium still lies to the left, but
there are more H+ ions compared to the
dissociation of alcohols.
As alcohols have a lower [H+ (aq)] in solution compared to water, alcohols are weaker acids
than water.
The inductive effect in alcohols
Electron-donating species such as alkyl groups push electrons into a covalent bond and are
said to have a positive inductive effect. In alcohols, the oxygen atom in the alkoxide ion is
bonded to an electron-donating alkyl group. This means that there is more electron density
on the O– atom. The alkoxide ion is, therefore, more likely to accept an H+ ion and form the
alcohol again.
2
Chemistry AS Notes 2023 Ethanol the biofuel
could it be the fuel
Organic Chemistry of the future?
Read
Hydroxy compounds: Alcohols
introduction to chapter 17
Alcohols are organic molecules containing at least one hydroxyl 3rd edition of the Chem
group, -OH. With one hydroxyl group substituted into an alkane to learn more.
molecule, the general formula is CnH2n + 1OH.
For alcohol molecules with three or more carbon atoms, the
position of the hydroxyl group is shown by inserting a number
to indicate which carbon atom is bonded to the OH group.
Classification of alcohols.
Alcohols are classified according to the number of carbon atoms attached to the carbon atom
bonded to the -OH group:
Primary alcohols are alcohols in which the carbon atom bonded to the -OH group is
attached to one other carbon atom (or alkyl group)
Secondary alcohols are alcohols in which the carbon atom bonded to the -OH group
is attached to two other carbon atoms (or alkyl groups)
Tertiary alcohols are alcohols in which the carbon atom bonded to the -OH group is
attached to three other carbon atoms (or alkyl groups)
Remember :
Characteristic distinguishing reactions:
Only primary and secondary alcohols can get oxidised when mildly oxidised with acidified
K2Cr2O7. Primary alcohols get mildly oxidised to aldehydes and secondary alcohols get
mildly oxidized to ketones. Tertiary alcohols do not undergo oxidation with acidified
K2Cr2O7.
Therefore, only the oxidation of primary and secondary alcohols will change the colour of
K2Cr2O7 solution as the orange Cr2O72- ions are reduced to green Cr3+ ions (figure 17.8).
1
, Acidity of alcohol
Alcohols have a low degree of
dissociation. This means, that when
dissolved in water, alcohol molecules
do not dissociate (split up) to a great
extent.
The position of the equilibrium lies to
the left, meaning that there are far more
alcohol molecules than RO– and
H+ ions.
When water dissociates, the position of
the equilibrium still lies to the left, but
there are more H+ ions compared to the
dissociation of alcohols.
As alcohols have a lower [H+ (aq)] in solution compared to water, alcohols are weaker acids
than water.
The inductive effect in alcohols
Electron-donating species such as alkyl groups push electrons into a covalent bond and are
said to have a positive inductive effect. In alcohols, the oxygen atom in the alkoxide ion is
bonded to an electron-donating alkyl group. This means that there is more electron density
on the O– atom. The alkoxide ion is, therefore, more likely to accept an H+ ion and form the
alcohol again.
2
