Alcohols, Phenols and Ethers 1 Flashcards
classification based on no of oh groups
Alcohols and phenols may be classified as mono–, di–, tri- or
polyhydric compounds depending on whether they contain one, two,
three or many hydroxyl groups respectively in their structures as
compounds containing Csp3-OH
Compounds containing C OH 3 − sp bond: In this class of alcohols,
the –OH group is attached to an sp3
hybridised carbon atom of an
alkyl group. They are further classified as follows:
Primary, secondary and tertiary alcohols: In these three types of
alcohols, the –OH group is attached to primary, secondary and
tertiary carbon atom
allylic alc
Allylic alcohols: In these alcohols, the —OH group is attached to
a sp
3
hybridised carbon next to the carbon-carbon double bond,
that is to an allylic carbon.
Benzylic alcohols:
In these alcohols, the —OH group is attached to
a sp3—hybridised carbon atom next to an aromatic ring. For
cpds containign Csp2-OH
These alcohols contain
—OH group bonded to a carbon-carbon double bond i.e., to a
vinylic carbon or to an aryl carbon. These alcohols are also known
as vinylic alcohols.
types of ethers
Ethers are classified as simple or symmetrical, if the alkyl or aryl
groups attached to the oxygen atom are the same, and mixed or
unsymmetrical, if the two groups are different. Diethyl ether,
C2H5OC2H5
, is a symmetrical ether whereas C2H5OCH3
and C2H5OC6H5
are unsymmetrical ethers.
explain bonding in Alcohols
In alcohols, the oxygen of the –OH group is attached to carbon by a
sigma (σ ) bond formed by the overlap of a sp
3
hybridised orbital of
carbon with a sp3
hybridised orbital of oxygen.
give reason: the bond angle in methanol/alkanol is much less than the normal tetrahedral angle
The bond angle in alcohols is slightly less than the tetrahedral
angle (109°-28′). It is due to the repulsion between the unshared
electron pairs of oxygen.
give reason: the C-O bond length in phenol is shorter
In phenols, the –OH group is attached to sp2
hybridised carbon of an aromatic ring. The carbon– oxygen bond
length (136 pm) in phenol is slightly less than that in methanol. This
is due to (i) partial double bond character on account of the conjugation
of unshared electron pair of oxygen with the aromatic ring (Section
11.4.4) and (ii) sp2
hybridised state of carbon to which oxygen is
attached.
give reason: the bond angle in ether is larger than the normal tetrahedral angle
In ethers, the four electron pairs, i.e., the two bond pairs and two
lone pairs of electrons on oxygen are arranged approximately in a
tetrahedral arrangement. The bond angle is slightly greater than the
tetrahedral angle due to the repulsive interaction between the two
bulky (–R) groups. The C–O bond length (141 pm) is almost the same
as in alcohols.
bpts of alcohols, phenols depend upon
The boiling points of alcohols and phenols increase with increase in the
number of carbon atoms (increase in van der Waals forces). In alcohols,
the boiling points decrease with increase of branching in carbon chain
(because of decrease in van der Waals forces with decrease in surface
area).
why is the bpt of alchohols significantly higher compared to bpt of other hydrocarbons
boiling points of alcohols and phenols
are higher in comparison to other classes of compounds, namely
hydrocarbons, ethers, haloalkanes and haloarenes of comparable
molecular masses. For example, ethanol and propane have comparable
molecular masses but their boiling points differ widely. The boiling
point of methoxymethane is intermediate of the two boiling points.
The high boiling points of alcohols are mainly due to the presence
of intermolecular hydrogen bonding in them which is lacking in ethers
and hydrocarbons.
what does solubility of alcohils and phenols
Solubility of alcohols and phenols in
water is due to their ability to form
hydrogen bonds with water molecules
as shown. The solubility decreases with
increase in size of alkyl/aryl (hydrophobic) groups. Several of the lower
molecular mass alcohols are miscible
with water in all proportions.
