Organic Chemistry (II) : Alcohols and Phenols - Nitrogen Compounds Flashcards
List the Physical Properties of Alcohols and Phenols
- Boiling Point
- Solubility in Water
- a) Comparison between alkanes and alcohols :
The boiling point of an alcohol is higher than that of an alkane of similar relative molecular mass since more energy is required to overcome the intermolecular hydrogen bonding in alcohols which is stronger than he dispersion forces in alkanes.
b) Comparison within the homologous series
Boiling point of alcohols increases as the number of carbon atoms increases. More e- more polarisable, dispersion forces gets stronger. - Alcohols are more soluble in water compared to their corresponding alkanes due to their ability to form hydrogen bonds with water molecules.
Solubility of alcohols decreases with increasing carbon number due to the increasing length of the homophobic non-polar hydrocarbon chain.
List and compare the chemical properties of alcohols and phenols.
1. Acidity
and
What effects the acidity?
- The more stable the conjugate base, the greater the extent of dissociation, resulting in a more acidic compound.
a) Phenol is more acidic than alcohol, since the negative charge on the phenoxide ion can be delocalised over the benzene rings and this stabilises the phenoxide ion. (So more likely to form)
b) Alcohols are weaker acids than water as alcohols have electron donating alkyl groups which intensify the negative charge over the benzene ring
Electron-withdrawing groups stabilise the conjugate bases formed by dispersing the negative charge on the alkoxide or phenoxide ion, resulting in increased acidity
(Eg. Halogens, NH2, NO2, COCH3, COOH, CN)
Electron-donating groups destabilise the conjugate bases formed by intensifying the negative charge on the alkoxide or phenoxide ion, resulting in decreased acidity
(Eg. Alkyl groups)
pH : Alcohol greater than H2O greater than phenol
.
Differences in esterification process between alcohol and phenol based on their reactivity?
Alcohols are stronger nucleophiles so it can react with carboxylic acids to form esters.
Phenoxides needs to react with acyl chlorides.
For higher yield, phenol is converted to the phenoxide salt by reacting with Na(s) or NaOH(aq) to form phenoxide ion, a stronger nucleophile, before reacting with acyl chloride
Distinguishing between
1. Alcohols and phenols
and
2. Primary and Secondary alcohols?
- Absence of fumes of HCl on adding PCl5 to phenols. C-O bond in phenol is very strong due to delocalisation of the lone pair of electrons on the oxygen over the benzene ring. No reactions in which the C-O bond breaks in phenols
- Primary to aldehyde
Secondary to ketone
then Tollens’ reagent
Oxidation of phenols
Phenols are not oxidised because they do not have a hydrogen atom on the hydroxyl-bearing carbon
Electrophilic substitution of phenols
Phenols are much more susceptible to nucleophilic attack since the presence of the -OH group highly activates the benzene ring towards nucleophilic substitution and no catalyst is required.
Delocalisation of lone pair of electrons of hydrogen into the benzene ring greatly increases the electron density in the benzene ring
List the Physical Properties of Aldehydes and Ketones
- Boiling Point
- Solubility in Water
- Boiling points : (Alcohols and carboxylic acids) is greater than (carbonyl) is greather than alkanes
a) Carbonyl (C=O) groups are polar, thus pd-pd interactions exists between molecules in aldehydes and ketones.
b) Alkane groups are non-polar, thus only have weak dispersion forces between molecules
c) Alcohols and carboxylic acids can form hydrogen-bonds between molecules, which are stronger and require more energy to overcome than pd-pd interactions in aldehydes and ketones - Solubility in water
Carbonyls have both polar and non-polar regions. They can act as solvents to both polar and non-polar solutes.
Similar to alcohols, higher members of carbonyl compounds containing more than 5 carbons or aromatic rings are insoluble in water
- Describe nucleophilic addition of HCN
- Explain the catalyst used
- Explain stereochemistry of Nucleophilic Addition
- . Explain relative reactivity of Aldehydes and Ketones in nucleophilic addition
- Memorise it
- a) Add trace amount of KCN
HCN is a weak acid, which only partially ionises in water to give CN-, very slow
trace KCN completely dissociates to provide free CN- ions
b) Add a small amount of base (NaOH)
H3O+ neutralised, which by Le Chatelier’s Principle, shifts the position of the equilibrium to the right such that the concentration of the CN- increases. Hence rate of reaction increases - Geometry surrounding the carbonyl carbon is planar. Nucleophile can attack from top or bottom in equal probability. Results in racemic mixture IF resulting sp3 carbon is chiral
- Aldehydes are more reactive than ketones in nucleophilic addition reactions for both steric and electronic reasons
a) Steric reason
Aldehydes 1 alkyl group, ketones have 2. Attacking nucleophile able to approach carbonyl carbon in aldehyde more readily as there is less steric hinderance
b) Electronic reason
Aldehydes have 1 electron donating alkyl group, ketones have two. Higher partial positive charge
Purpose of carbonyl reactions with
- (2,4-DNPH)
- K2Cr2O7/ dilute H2SO4, heat
- Tollens’ Reagent
- Fehling’s Solution
- Tri-Iodomethane (Iodoform) Test
- Test for presence of carbonyl groups
2,4-DNPH turns from a yellow solution to an orange ppt - Test to differentiate aldehydes and benzaldehydes from ketones
(Carboxylic acid formed)
Orange to green - Test for all aldehydes (including benzaldehydes)
Silver metal/mirror formed - Test for aliphatic aldehydes only
blue solution to reddish-brown ppt - Test for ethanal or methyl ketones
Aqueous I2 with NaOH(aq), warm
Yellow crystals of CHI3 (iodoform) formed
List the Physical Properties of Carboxylic acids
- Boiling Point
- Solubility
- a)Higher than alkanes due to strong intermolecular hydrogen bonding
Higher than alcohols since the electron withdrawing C=O group causes the O-H bond to be more polarised in the carboxylic acid
b) Dimerise in the liquid state and vapour phase through formation of 2 hydrogen bonds between each pair of molecules, effectively doubling the Mr. Stronger dispersion forces - a) Soluble in non-polar solvents. Exists as dimers
b) Carboxylic acids with 4 or fewer carbons are soluble in water. Hydrogen bonding
Explain the acidity of carboxylic acids
pH lower than phenoxide since the negative charge is delocalised equally over the two electronegative oxygen atoms whereas for phenoxide, the negative charge is only slightly delocalised into the ring. Therefore more stable.
Explain why carboxylic acids undergo nucleophilic acyl substitution
- Electronegative O results in the partial positive charge on the C=O carbon
- The C=O carbon has a planar geometry and is relatively unhindered which makes it susceptible towards nucleophilic attack
Account of the difference in the relative rate of hydrolysis of acyl chloride, alkyl chlorides and aryl chlorides
Rate of hydrolysis : Acyl is the fastest followed by alkyl and then aryl
a) Steric Factor
- Planar acyl carbon makes it less hindered for the nucleophile to attack
- Tetrahedral geometry around chloro-substituted carbon is relatively more hindered
- Bulky aromatic ring with electron rich pi clouds above and below the benzene ring repels nucleophile
b) Electronic Factor
- Highly electron deficient carbon on C-Cl due to 2 electronegative atoms Cl and O
- Less electron deficient carbon on C-Cl due to polarisation by one electronegative Cl atom only
- Least electron deficient carbon on C-Cl due to the delocalisation of the lone pair of electrons on Cl into the benzene ring, imparting partial double bond character to the C-Cl bond in aryl chlorides, strengthening the bond
