Chapter 16 - Hydroxy Compounds

Question 1

Why are the boiling points of alcohols and phenols significantly higher than that of their corresponding alkanes?

Answer 1

while the strength of dispersion forces in the alcohol and in the alkane is about the same due to the similar sizes of electron clouds, more energy is required to overcome the intermolecular hydrogen bonding in alcohols, which is stronger than the dispersion forces in alkanes.

Question 2

Why does the boiling point of alcohols increase as it alkyl chain length increases?

Answer 2

The strength of intermolecular hydrogen bonding remains largely similar for all alcohols, but the size of electron cloud increases as the length of alkyl chains increases. More energy is thus required to overcome the stronger dispersion forces between the alcohol molecules.

Question 3

Why are alcohols more soluble in water compared to their corresponding alkanes?

Answer 3

They have the ability to form hydrogen bonds with water molecules.

Question 4

Why does the solubility of alcohols in water decrease as it alkyl chain length increases?

Answer 4

1) As the length of the alkyl chain increases, the strength of the dispersion forces between alcohol molecules increases. Energy released from hydrogen bonding between the alcohol molecule and water is less able to overcome the increasingly stronger dispersion forces, as well as the existing hydrogen bonding in water.
2) hydrogen bonding with water is disrupted by the larger non-polar alkyl chain.

Question 5

Can phenols dissolve in water?

Answer 5

Phenols are only moderately soluble in water due to their large non-polar aryl group, but dissolve completely when warmed.

Question 6

How does the reactivity of the –OH group aid alcohols in undergoing reactions? (2)

Answer 6

1) by breaking the C–OH bond, the alcohol can undergo elimination reactions or nucleophilic substitutions.
2) by breaking the O–H bond, it releases H+ ions, converting alcohols to alkoxides. This allows the alcohol to behave as a Bronsted acid.

Question 7

Compare the acidity of water, alcohols and phenols.

Answer 7

Phenols > Water > Alcohols

Question 8

What does the strength of acids depend on?

Answer 8

The stability of the conjugate base formed after dissociation. When a more stable conjugate base is formed, it is less likely to accept a proton to re-form the acid. Hence, the extent dissociation is greater, resulting in a stronger acid.

Question 9

Why are alcohols weaker acids than water?

Answer 9

Alkyl groups are inductively electron-donating and will intensify the negative charge on the alkoxide ion. The alkoxide ion will have a greater tendency to accept a proton to re-form the alcohol molecule. Thus, the alkoxide ion is destabilised, and the deprotonation (removing H+) of the alcohol does not take place as easily. (length of alkyl chain does not have much impact on acidity)

Question 10

What are the effects of substituents on acidity of alcohols? (3)

Answer 10

1) electron-withdrawing groups: help to stabilise the alkoxide ion by dispersing the negative charge on the alkoxide ion, thus promoting the ionisation of the alcohol.
2) electron-donating groups: decrease the acidic strength of alcohols by intensifying the negative charge on the alkoxide ion, destabilizing the alkoxide ion.
3) distance: the nearer the substituent, the stronger its effect.

Question 11

Why are phenols stronger acids than water?

Answer 11

In the phenoxide ion, a p orbital of the oxygen atom is able to overlap with the pi electron cloud of the benzene ring. The negative charge is able to delocalise into the benzene ring to some extent, thus the phenoxide ion is stabilized by resonance. The phenoxide ion is less likely to accept a proton to re-form phenol, thus making it stronger acids than water and alcohols.

Question 12

What are the effects of substituents on acidity of phenols? (2)

Answer 12

1) electron-withdrawing groups: enables the ring to withdraw more electron density from the oxygen, thus stabilising the phenoxide ion further and promoting the ionisation of phenol.
2) electron-donating groups: decreases the acid strength of phenols by reducing the delocalisation of the negative charge on the oxygen atom into the ring or by intensifying the negative charge on the phenoxide ion, thus destabilising it.

Question 13

What is 1 reagent, 1 condition and 1 observation of the acid-metal reaction of alcohols and phenols?

Answer 13

reagent: group 1 metal solid
condition: room temperature
observations: slow effervescence of hydrogen gas for alcohols, rapid effervescence for phenols

Question 14

What is 1 reagent, 1 condition and 1 observation of the acid-metal reaction of phenols? (phenol only)

Answer 14

reagent: NaOH (aq)
Condition: room temperature
observations: cloudy mixture dissolves to form colourless homogenous solution

Question 15

What are leaving groups?

Answer 15

a molecular fragment that departs with a pair of electrons in heterolytic bond cleavage. weaker bases are generally better leaving groups.

Question 16

How does nucleophilic substitution with hydrogen halides occur for alcohols?

Answer 16

since the hydrogen halides like HCl are stronger acids than water, Cl- is a weaker conjugate base than OH-. Thus, Cl-, being a better leaving group than OH-, is readily substituted by OH- to form alcohols.

Question 17

Why are reagents such as acid of SOCl2 needed to convert alcohols to halogenoalkanes?

Answer 17

OH- is a poorer leaving group than Cl-. These reagents first convert the OH- group into a better leaving group before the halide ion attacks.

Question 18

Why do primary and secondary alcohols react with concentrated HCl slower than tertiary alcohols?

Answer 18

The reaction takes place via the SN1 mechanism, forming a carbocation intermediate. The rate of reaction depends on the stability of the carbocation formed. Since tertiary alcohols have 3 electron-donating alkyl groups, it is the most stable. Thus, it reacts faster than primary and secondary alcohols and do not require ZnCL2 and heat.

Question 19

Why are phenols inert to nucleophilic substitution, where –OH is replaced by a halide? (2)

Answer 19

1) the lone pair of electrons on the O atom in phenol delocalises into the benzene ring, resulting in partial double bond character in the C–O bond. The C–O bond strength in phenols is higher than that in an alcohol, making it more difficult to break.
2) sterically, the approach of a nucleophile to the rear side of C–O bond in phenols is hindered by the benzene ring. The pi electron cloud of the benzene ring will also repel the lone pair of electrons on the approaching nucleophile.

Question 20

Why tertiary alcohols and phenols cannot be oxidised using K2Cr2O7 or KMnO4?

Answer 20

The alcohol must have at least one hydrogen atom bonded to the C atom bearing the –OH group to undergo oxidation using these reagents.

Question 21

How can alcohols be prepared? (4)

Answer 21

1) electrophilic addition of water or steam to alkenes
2) nucleophilic substitution of halogenoalkanes
3) reduction of aldehydes and ketones (to yield primary and secondary respectively)
4) reduction of carboxylic acids (primary alcohols only)