ALCOHOLS, PHENOLS AND ETHERS

Question 1

The carbon- oxygen bond length (136 pm) in phenol is slightly less than that in methanol.

Answer 1

partial double bond character (ii) sp2 hybridized state of carbon to which oxygen is attached.

Question 2

The bond angle in ethers is slightly greater than the tetrahedral angle.

Answer 2

repulsive interaction between the two bulky (-R) groups.

Question 3

The boiling points of alcohols decrease with increase of branching in carbon chain.

Answer 3

decrease in van der Waals forces with decrease in surface area.

Question 4

Ethanol has higher boiling point than methoxymethane.

Answer 4

presence of strong intermolecular hydrogen bonding between ethanol molecules.

Question 5

The bond angle in alcohols is slightly less than the tetrahedral angle (109.28’)

Answer 5

This is due to the repulsion between the unshared electron pairs of oxygen.

Question 6

Alcohols are soluble in water.

Answer 6

ability to form hydrogen bonds with water molecules.

Question 7

neopentyl alcohol has a lower boiling point than n-pentyl alcohol.

Answer 7

decrease in the van der Waals forces with the decrease in surface area.

Question 8

n-hexanol is not soluble in water.

Answer 8

In n-hexanol, alkyl group is quite large. As a result it resembles the
hydrocarbons more closely and is not soluble in water. The extent of H-bonding between n-hexanol and water is very less.

Question 9

ter-butyl alcohol is more soluble in water than n-butyl alcohol.

Answer 9

In ter-butyl alcohol, the alkyl group is highly branched and hence approaches almost spherical shape.
As a result, non-polar part of the molecule becomes less prominent and hence it can have stronger intermolecular Hydrogen bonds with water molecules through its O-H group.
On the other hand n-butyl alcohol has predominantly non-polar character.

Question 10

Alcohols are acidic in nature.

Answer 10

presence of a polar O-H bond. Oxygen is more electronegative than H2 and therefore it withdraws the shared pair of electron
O-H bond becomes weak and loses a proton H+

Question 11

Comparison of acidic strength of different types of alcohols: the order of the acidic strength of alcohols follows the order $1^o$> $2^o$ >$3^o$

Answer 11

due to the polar nature of O-H bond.
An electron-releasing group (-CH3,-C2Hs) increases electron density on O -> decrease the polarity of O-H bond.
This decreases the acid strength.

Question 12

Water is stronger acid than ethanol.

Answer 12

Ethoxide ion is less stable than hydroxide ion due to concentration of negative charge on oxygen due to electron releasing inductive effect of ethyl group. Hence, equilibrium in equation (ii) is more in forward direction producing more H3O+ ions.

Question 13

Phenols are acidic.

Answer 13

presence of polar O-H bond.
The hydroxyl group in phenol is directly attached to the sp2 hybridised carbon of benzene ring which acts as an electron withdrawing group. Due to the higher electronegativity of sp2 hybridised carbon of phenol to which-OH is attached, the electron density on oxygen decreases. This increases the polarity of O-H bond and results in an increase in ionization of phenols. Hence H+ ion can be easily released.
The delocalisation of negative charge (structures I-V) makes phenoxide ion more stable and favours the ionisation of phenol. Although there is also charge delocalisation in phenol, its resonance structures have charge separation due to which the phenol molecule is less stable than phenoxide ion. So Phenol releases H+ ion to form Phenoxide ion. Thus phenol is acidic in nature.

Question 14

Alcohols are less acidic than Phenols

Answer 14

The ionisation of an alcohol and a phenol takes place as follows:
i) Due to the higher electronegativity of sp2 hybridised carbon, of phenol to which -OH is attached, electron density decreases on oxygen. This increases the polarity of O-H bond and results in an increase in ionisation of phenols than that of alcohols.

ii) Stabilities of alkoxide and phenoxide ions: In alkoxide ion, the negative charge is localised on oxygen while in phenoxide ion, the charge is delocalised. The delocalisation of negative charge (structures I-V) makes phenoxide ion more stable and favours the ionisation of phenol.

Question 15

o-nitrophenol is more acidic than Phenol.

Answer 15

-NO2 group being an electron withdrawing group, withdraws the electrons and disperse the negative charge of the phenoxide ion, thus making the phenoxide more stable. Thus increasing the acid strength.

Question 16

Cresols are less acidic than phenol.

Answer 16

Alkyl group being an electron releasing group, intensify the charge on the phenoxide ion making it unstable, thus decreasing the acid strength.

Question 17

o-nitrophenol is more steam volatile than p-nitrophenol.

Answer 17

o-nitrophenol is steam volatile due to intramolecular hydrogen bonding while p- nitrophenol is less volatile due to intermolecular hydrogen bonding which causes the association of molecules.

Question 18

ortho nitrophenol is more acidic than ortho methoxyphenol.

Answer 18

Due to the strong -R effect and -1 effect of NO2 group, electron density on the oxygen atom decreases. This increases the polarity of O-H bond and hence results in the release of H+ ion very easily. Also o-nitrophenoxide ion is stabilized by
resonance.
On the other hand due to +R effect of the -OCH, group, the electron density on the oxygen atom increases. This decreases the polarity of the O-H bond and this makes the loss of proton difficult.

Question 19

Phenols have a smaller dipole moment than methanol.

Answer 19

Due to electron withdrawing effect of phenyl group, the C-O bond in phenol is less polar, whereas in case of methanol the methyl group has electron releasing effect and hence C-O bond in it is more polar.

Question 20

The halogenation of Phenol takes place in the absence of a Lewis acid, such as FeBr3

Answer 20

In case of phenol, the polarization of bromine molecule takes place even in the absence of Lewis acid. It is due to the highly activating effect of -OH group attached to the benzene ring.

Question 21

Preparation of ethers by acid dehydration of secondary or tertiary alcohols is not a suitable method.

Answer 21

Acid catalyzed dehydration of primary alcohols to ethers take place by SN2 reaction involving nucleophilic attack by the alcohol molecule on the protonated alcohol molecule.

However under these conditions, $2^o$ and $3^o$ alcohols give alkenes rather than ethers. This is because of the steric hindrance, nucleophilic attack by the alcohol on the protonated alcohol molecule does not take place. Instead of this, the protonated $2^o$ and $3^o$ alcohol loses a molecule of water to form stable $2^o$ and $3^o$ carbocations. These carbocations then prefer to lose a proton to form alkene rather than undergoing nucleophilic attack by alcohol molecule to form ethers.