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Alcohols
Functional group: -OH.
Hydroxyl group covalently bonded to an alkyl chain or benzene ring (called
phenols)
Many naturally occurring in nature: glucose C6H12O6, glycerol C3H8O3
Most common alcohol is ethanol, which has been produced for thousands of
years – Egyptian workers given beer rations! And governments have taxed
alcoholic drinks for years – a good source of revenue.
Two methods of production – fermentation,
from ethene.
Alcohols can be classed as primary, secondary, tertiary and this is important in
the reactions they undergo. (more later)
Alcohols with one -OH group called monohydric (alcohol)
Alcohols with two -OH groups called dihydric (ethanol-1,2-diol / antifreeze)
Alcohols with three -OH groups called trihydric (propane-1,2,3-triol / glycerol)
Alcohols with many -OH groups per molecule is called polyhydric (glucose)
Physical props
B/pt of an alcohol increases with length of hydrocarbon chain and number of –OH
groups attached.
Two types of forces between alcohol molecules – weak induced dispersion forces
and hydrogen bonding.
There are more induced dispersion forces between longer alkyl chains – primary
alcohols with longer chains can have higher b/pts than secondary or tertiary
alcohols of the same molecular mass due to the bulky alkyl groups not allowing
the chain to lie close to each other therefore less induced dipole-dipole
interactions.
Hydrogen bonding exists between the oxygen atoms in the hydroxyl group and
the hydrogen atoms in the hydroxyl groups of neighbouring molecules.
Solubility
Methanol, ethanol, propan-1-ol all completely miscible in water due to the
hydrogen bonding hydroxyl groups in water and the hydroxyl groups in alcohol.
However as the length of the hydrocarbon chain increases, solubility decreases,
due to more dispersion forces established between the alkyl groups of the alcohol
molecules so they are more attracted to each other than the water molecules.
Ethanol has polar bonds in its molecule: C - O and O – H. Oxygen is much more
electronegative than carbon and hydrogen so a permanent dipole is present in
these bonds. A molecule of ethanol can form dispersion forces, dipole-dipole
forces and hydrogen bonds between its molecules and interactions with other
molecules, which makes it an important solvent.
Ethanol is a solvent for polar and non-polar substances. The polar part dissolves
polar substances by forming dipole-dipole interactions or hydrogen bonds with
the substances. The hydrocarbon tail dissolves non-polar substances by forming
dispersion forces with the substance.
It is used in industrial processes such as; production of perfumes, varnishes,
adhesives, plastics. It is used as a solvent in medicines. This is very useful if the
medicine does not dissolve in water it can dissolve in ethanol and be taken in a
liquid form. (see text pg 31).