Unit 11 – Alcohols, Phenols and Ethers Download

Unit 11 – Alcohols, Phenols and Ethers.
Give reasons for the following:
1. The C-O-H bond angle in alcohols is slightly less than the tetrahedral angle (109 28’)
A. Due to repulsion between the unshared pairs of electrons of oxygen (lp-lp repulsion).
2. The C-O bond length in phenol is slightly less than in methanol and other alcohols.
A. This is due to (i)partial double bond character on account of the conjugation of
unshared electron pair of oxygen with the aromatic ring and (ii)sp2 state of C to which O
is attached.
3. The C-O-C bond angle in ether is slightly greater than the tetrahedral angle.
A. Due to repulsion between the two bulky (-R) groups.
4. Boiling pints of alcohols and phenols are higher than those of hydrocarbons, ethers,
haloalkanes and haloarenes of comparable molecular masses.
A. This is due to the presence of intermolecular H-bonding in alcohols and phenols.
5. Lower homologues of alcohols and phenols dissolve in water but higher ones are less
A. Lower members dissolve due to their ability to form H-bonds with water. Higher
members are insoluble because with increase in size of R (alkyl/aryl group), hydrophobic
effect of these groups reduce solubility.
6. Alcohols react both as nucleophiles and electrophiles
A. Due to polarity of both C-O bond and O-H bond. When O-H bond is broken, alcohols
react as nucleophile. When C-O bond breaks, they react as electrophiles.
7. Acid strength of alcohols is in order 1 > 2 > 3
A. Acid strength depends on polarity of O-H bond. Electron releasing groups (R-CH3,
R-C2H5 etc) increase electron density on oxygen and reduce polarity of O-H bond.
CH 2OH 
C  OH
8. Alcohols are weaker acids than water.
A. Electron releasing group R reduces polarity of O-H bond
9. Alcohols also act as bases.
A. Due to presence of unshared electron pairs (lone pairs) on oxygen atom. They act as
proton acceptors (Bronsted base).
10. Phenols are more acidic than alcohols
i) C of C-O bond in phenol is sp2 hybridized, so C-O bond is less polar. This increases
polarity of O-H bond where as e- releasing R increasing polarity of C-O bond and reduces
polarity of O-H bond in alcohols.
ii) The phenoxide ion
where as alkoxide ion RO- is not.
formed after loss of H+ is stabilized by resonance
R-OH ⇌ R-O- + H+
11. The order of acid strength for substituted phenols is
A. In substituted phenols, presence of electron withdrawing groups like –NO2 especially
at o- and p- positions enhances acid strength due to greater delocalization of negative
charge of phenoxide ion. Therefore, they facilitate the release of H+ ion.
On the other hand electron releasing groups decrease acid strength as they reduce
stability of phenoxide ion.
12. pKa value of cresols is higher than that of phenol
A. Lower the pKa value, stronger the acid. pKa= -log Ka or higher the Ka value stronger
the acid.
Cresol (o, m or p isomer)
has e- releasing -CH3 which destabilizes
phenoxide ions and reduces acid strength.
13. Esterfication is carried out in the presence of small amount of conc. H2SO4.
A. Esterfication is a reversible process. To prevent the backward reaction, water is
removed a soon as it is formed by conc. H2SO4 which is a dehydrating agent.
14. Production of ester from RCOCl is carried out in the presence of a base (pyridine).
A. The base neutralizes the acid, HCL formed and shifts the equilibrium to the right and
increases the amount of product formed.
15. The order of reactivity of alcohols towards Lucas reagent (HCl and ZnCl2) is
A. In the reaction the C-O bond is broken, which is the most polar for 3o alcohols, due to
electron releasing inductive effect of three alkyl groups.
RCl + H2O
16. The ease of dehydration of alcohols to form alkenes is in order 3>2>1.
A. In the rate determining step of dehydration carbocation is formed. Tertiary
carbocations are more stable and therefore are easier to form than secondary and primary
17. Tertiary alcohols resist oxidation.
A. Oxidation of 1 and 2 alcohols involve cleavage of O-H and C-H bond. Since 3
alcohols do not have C-H bond, they resist oxidation.
18. Phenols undergo electrophilic substitution in o- and p- positions
A. –OH attached to benzene ring activates it towards electrophilic substitutions at o- and
p- positions as these positions became electron rich due to resonance effect caused by
-OH group.
19. When phenol is heated with bromine water 2,4,6-tribromophenol is formed.
A. Due to highly activating effect of –OH group, the polarization of Br2 molecule takes
place even in the absence of Lewis acid like FeBr3.
20. Preparation of ethers by dehydration of alcohols is suitable from primary alcohols
A. Reaction follows SN2 mechanism with primary alcohol and ether is formed at 413K.
Whereas in secondary and tertiary alcohols, it follows SN1 mechanism and elimination
competes over substitution and due to steric hindrance of alkyl group: alkenes are more
easily formed.
21. The following reactants are not appropriate for the preparation of tert-butylethylether.
C2H5ONa +
A. The major product of the above reaction is 2-methylprop-1-ene
It is because sodium ethoxide C2H5ONa is a strong nucleophile as well as a strong base.
Thus elimination predominates over substitution.
22. Anisole is prepared from sodium phenoxide and methyl halide and not halobenzene
and sodium methoxide.
A. In halobenzene
the C-X bond is stabilized by resonance, has partial double
bond character and is difficult to break.
23. Ethers have much lower boiling points than alcohols of comparable molecular mass.
A. Though ethers are polar they are low in polarity and there are no intermolecular Hbonds unlike alcohols.
24. Lower ethers are miscible in water, but higher homologues are immiscible.
A. Like alcohols, lower homologues can from H-bonds with water. With increase in size
of alkyl groups (R), their hydrophobic character increases and solubility reduces.
25. Reaction of alkyl aryl ether with HI (e.g. Anisole) gives phenol and alkyl iodide
(methyl iodide) and not halobenzene and alcohol.
+ HI 
+ CH3I
A. The C-O bond (aryl-oxygen bond) is resonance stabilized and has partial double bond
character and is difficult to break; alkyl-oxygen bond is easier to break.
26. The order of reactivity of hydrogen halides with ether is HI>HBr>HCl.
A. HI bond is weakest and easiest to break.
27. In the reaction of mixed ether with HI, when primary or secondary alkyl groups are
present, it is the lower alkyl group that forms alkyl iodide. Whereas, if one alkyl group is
tertiary, the halide formed is tertiary halide.
A. In the case of primary and secondary alkyl groups reaction occurs by SN2 mechanism
and iodide attacks the R with less steric hindrance, while in ethers with tertiary alkyl
groups, it occurs by SN1 mechanism and more stable tertiary carbocation is formed which
combines with I- to form tertiary Iodide.
28. In alkyl aryl ethers, electrophilic substitution occurs at o- and p- positions.
A. The alkoxy group -OR is an activating group (e- releasing) and due to resonance,
electron density is high in o- and p- positions.
29. o-nitrophenol is steam volatile while p-nitrophenol is not.
A. There are intermolecular H-bonds in p-nitrophenol while in o-nitrophenol there are
intramolecular H-bonds. Therefore, intermolecular forces are weaker in o-nitrophenol.
Solve the following give reason questions of the text book: page 343-345
Q11.4, 11.5, 11.15 11.16 11.22 11.27 11.29