hydroxy compounds

Question 1

why do alcohols have higher boiling points than alkanes with relatively smilar molecular mass?

Answer 1

More energy is required to overcome the stronger hydrogen bonds between alcohol molecules as compared to the weaker instantaneous dipole-induced dipole interactions between alkane molecules.

hydrogen bonding

Question 2

why do alcohols with longer alkyl chains have higher boiling points?

Answer 2

a longer alkyl chain would mean a larger electron cloud size, which can be polarised to a greater degree. Hence, the molecules would have stronger intermolecular id-id interactions between the alkyl groups.

Question 3

why do straight-chain alcohols generally have higher boiling points than their brnached isomers?

Answer 3

branching decreases the surface area of interaction between molecules and hence the strength(+extensiveness) of intermolecular id-id interactions.

Question 4

why are alcohols with shorter carbon chains miscible with water but solubility decreases as number of carbons increases?

Answer 4

1. the -OH group is polar and can form hydrogen bonds with water molecules
2. the alkyl group is non-polar and it cannot form hydrogen bonds with water molecules. Instead, id-id interactions exist between alkyl groups
3. solubility of alcohols with longer carbon chains decreases because as the alkyl group/chain becomes larger/longer; the id-id interactions between the alkyl groups become predominant and these non-polar chains interfere with the hydrogen bonding between the -OH groups and water molecules.

Question 5

why do alcohols have a higher pKa value compared to water, thus considered neutral compounds?

Answer 5

higher pKa means lower Ka
strength of acid depends on the stability of its conjugate base
- a less stable conjugate base would mean that the acid dissociates to a smaller extent, producing less H+ ions, causing POE to lie more to the left and hence the acid is weaker.

conjugate base of water is OH- and that of ROH is RO-. For RO-, presence of electron-donating alkyl group increases the intensity of negative charge on the oxyfen atom in the alkoxide RO- anion. RO- is less stable than OH-. Hence, ROH will have a smaller extent of dissociation and is a weaker acid than water.

the bulky alkyl group also interferes with the formation of ion-dipole interactions between the alkoxide ion and water; leading to a lower degree of stabilisation of the anion.

Question 6

how to convert alkene into alcohol?

Answer 6

electrophilic addition
- H2O(g), H3PO4 catalyst

Question 7

how to convert RX into ROH?

Answer 7

nucleophilic substitution:
- NaOH(aq), heat

OH is a poor leaving group, so for any other reactions to take place, we need to convert ROH into RX first.

Question 8

how to convert aldehyde into alcohol? what type of alcohol is produced?

Answer 8

primary alcohol

reduction:
- LiAlH4 in dry ether OR
- H2(g) with Ni catalyst

Question 9

how to convert ketone into alcohol? what type of alcohol is produced?

Answer 9

secondary alcohol

reduction:
- LiAlH4 in dry ether OR
- H2(g) with Ni catalyst

Question 10

how to convert Carboxylic acid into alcohol? what type of alcohol?

Answer 10

primary alcohol

reduction:
- LiAlH4 in dry ether

Question 11

why must LiAlH4 be in dry ether?

Answer 11

LiAlH4 reacts with water, and hence, the presence of water destroys the reducing agent. thus, reduction must be done in anhydrous conditions.

Question 12

how to convert alcohol into alkene?

Answer 12

elimination:
- concentrated H3PO4 catalyst, heat

Question 13

how to convert alcohol into RX?

Answer 13

nucleophilic substitution:
- HX OR
- PCl5 for RCl(chemical test)

when using PCl5 as chemical test, white fumes of hydrogen chloride will be observed.

Question 14

what is the acid-metal displacement/redox reaction?

Answer 14

ROH + Na(s) = R-O- Na+

reagent and condition:
- Na(s)

Question 15

how to convert alcohol into ester?

Answer 15

condensation:
1.carboxylic acid concentrated H2SO4 catalyst, heat(reversible reaction)
-H removed from alcohol, OH removed from acid to form H2O
2. acyl chloride (RCOCl)(irreversible reaction)
-H removed from alcohol, Cl removed from acyl chloride to form HCl

Question 16

how to name esters?

Answer 16

1st part of the name comes from alcohol and second part of name comes from carboxylic acid/acyl chloride.

e.g. methyl ethanoate (methanol + ethanoic acid/ethanoyl chloride)

Question 17

what can primary alcohols oxidised to?

Answer 17

chemical test
1. aldehydes:
- acidified K2Cr2O7, heat with distillation
- remove H from OH group and H bonded to C atom to form H2O

2. carboxylic acid
   - acidified KMnO4/K2Cr2O7, heat under reflux
   - remove 2 Hs bonded to carbon atoms and add an O bonded to C(double bond) to form H2O

for both cases, purple KMnO4 decolourises

Question 18

what can secondary alcohols be oxidised to?

Answer 18

chemical test
ketones:
- acidified K2Cr2O7, heat with distillation
-** remove 2Hs bonded to carbon and add a double-bonded O to the carbon atoms to form H2O**.

purple KMnO4 decolourises

Question 19

what do tertiary alcohols oxidise to?

Answer 19

do not oxidise

chemical test
purple KMnO4 remains purple

Question 20

describe the trio-iodomethane rest

Answer 20

only works for structures with ** H, CH3, OH and an R group(alkyl gorup/H atom) bonded to a central carbon.

reagents and conditions:
- warm alkaline aqueous iodine

test(chemical test):
-oxidation
- add aqueous iodine in sodium hydroxide to a solution of alcohol in a test tube and warm using a water bath.

Observations:
- “decolourisation” of brown iodine solution seen
- pale yellow precipitate of tri-iodomethane(CH3I) will be observed.

iodine can be decolourised by sodium hydroxide via disproportionation reaction, without the presence of the alcohol. Hence, the formation of pale yellow ppt is a more confirmatory observation.

Question 21

why do phenols exist as crystalline solids at room temperature and has a high boiling point?

Answer 21

boiling involves the breaking of the strong hydrogen bond between phenol molecules and hence, a large amount of energy is required.

Question 22

why are phenols not very soluble in cold water but soluble in hot water?

Answer 22

although the -OH group is polar and can form hydrogen bonds with water molecules, the presence of the large hydrophobic benzene ring reduces its solubility in water.

Question 23

why are phenols stronger acids than water?

Answer 23

conjugate base of water: OH-
conjugate base of phenol: C6H5O-

the p orbital of the oxygen atom overlaps with the pi electron cloud of the ring, resulting in the delocalisation of the negative charge on oxygen into the ring, reducing the intensity of the negative charge.

Hence, C6H5O- is more stable than OH- due to charge delocalisation, resulting in C6H5OH to have a larger extent of dissociation and hence is a stronger acid.

Question 24

how to convert phenol into phenoxide?

Answer 24

1. Acid-metal displacement/redox reaction
   - Na(s)
   - forms sodium phenoxide and H2(g)
2. Acid-base reaction/ neutralisation
   - NaOH(s)
   - forms sodium phenoxide and H2O
   phenol does not react with bases such as sodium carbonate or hydrogencarbonate as it is a weaker acid than carbonic acid

Question 25

how to form ester from phenol? Justify reagents used.

Answer 25

condensation reaction: - NaOH(aq), followed by acyl chloride(ROCl) Phenol is a weaker nucleophile than alcohol as its lone of electrons on oxygen is able to delocalise into the benzene ring. Hence, the lone pair is less available for the condensation reaction. Hence, the phenoxide ion must first be produced via an acid-base reaction as it is a stronger nucleophile.

Question 26

for phenols, why does the substitution of a halogen to replace the hydroxy group not happen?

Answer 26

this substitution involves the breaking of the C-O bond. this does not happen for phenol. the p orbital of the O atom overlaps with the pi electron cloud of the ring reuslting in the C-O bond having a partial double bond character. hence, the amount of energy required to break the C-O bond is higher than a normal C-O bond and hence, the substitution does not happen unless the reactants are subjected to very vigorous conditions.

Question 27

why are reactions involving the benzene ring of phenols less harash compared to that of benzene?

Answer 27

the benzene ring in phenol is highly susceptible towards electrophilic substitution. the lone pair of electrons on the oxygen atom delocalises into the benzene ring, increasing the electron density of the benzene ring.

Question 29

describe reaction of phenols with dilute HNO3.

Answer 29

electrophilic substitution: - a mixture of 2-nitrophenol and 4-nitrophenol plus H2O as side product

Question 30

describe reaction of phenol with aqueous bromine, Br2(aq).

Answer 30

**chemical test** electrophilic substitution - Br substituted at 2,4,6th position - side product is HBr. observation(s): - orange aqueous bromine solution which is rapidly decolourised - white precipitate of 2,4,6-tribromophenol will be observed.

Question 31

what is the other chemical test for phenols?

Answer 31

**out of syllabus** test: - add aqueous neutral iron(III) chloride solution to phenol - a violet complex will be observed.