Topic 17: Organic Chemistry II

What is stereoisomerism?

Same structural formula but different arrangement of atoms in space

2 types of stereoisomerism

Geometrical and optical

What is optical isomerism?

An example of stereoisomerism where two molecules are non superimposable mirror images due to a chiral centre

- different arrangement of atoms in space

What are two optical isomers called?

enantiomers

When does optical isomerism occur?

When a carbon atom is bonded to four different groups (chiral)

What is a chiral carbon

Carbon atom with four different groups attached

What is meant by the term chiral?

two isomers which are non-superimposable

Physical and chemical properties of enantiomers?

Nearly identical, except rotation of plane-polarized light and reactivity in chiral environments

enantiomer effect on light

rotates in opposite directions but by same amount

What is a racemic mixture?

50/50 mixture of two enantiomers

racemic mixture in light

will not rotate polarised plane light

When will a racemate be formed?

in a reaction mechanism when a trigonal

planar reactant or intermediate is approached from both sides by an attacking species e.g SN1 mechanisms

Probability of forming specific enantiomer

50/50 even chance

What substances normally form chiral products ?

achiral reactants

Which mechanism (SN1 or 2) will form a racemic mixture ?

SN1

How does the SN1 mechanism form racemates

1) a halogen breaks away from halogenoalkane to form carbocation

2) carbocation is planar and so can be attacked from above and below

3) equal chance of direction therefore forms racemate

Why won't we form a racemate in SN2?

occurs via transition state which is not a planar molecule

What is produced if a chiral reactant undergoes SN2?

the opposite enantiomer will form and so light is rotated in the opposite direction

- as the nucleophile will always attack on the opposite side of leaving group

What is a carbonyl?

C=O

Why do aldehydes and ketones have lower boiling points than alcohols?

- they don't form hydrogen bonds

- only london and permanent dipole bonds due to polarity within the molecules

- therefore less energy is required to overcome intermolecular forces

Why can some aldehydes and ketones dissolve in water?

they form hydrogen bonds with water

- NOT ALONE

How do aldehydes and ketones form hydrogen bonds with water?

lone pair of electrons on O in C=O can form bond with hydrogen atoms in water

Which size aldehydes and ketones can dissolve in water ?

only smaller ones

- larger ones have large hydrocarbon components that are non polar and won't dissolve

- larger also form strong london forces that can become stronger than hydrogen bonds

3 ways to test for aldehydes/ketones

1) Fehlings

2) Tollens

3) Acidified dichromate ions

How can acidified dichromate distinguish between aldehyde and ketones ?

can oxidise aldehydes but not ketones

- so turns orange to green in aldehydes

- remains orange with ketone

Cr2O7 -2 is reduced to Cr 3+

- heat under reflux

How does Tollen's reagent distinguish between aldehydes and ketones?

add tollens to aldehyde/ketone and place in water bath

- tollens is reduced to silver in aldehydes which coats inside of flask

- no change with ketones

How is tollens formed ?

mix aqueous ammonia with silver nitrate

Aldehyde and Tollens equation

Ag(NH3)2 + + e- -> Ag + 2NH3

How does Fehling's test work?

The aldehyde reduces the blue copper (II) ions to brick-red copper (I) oxide

- ketone has no effect remaining blue

- heat in alkaline conditions

Product of reducing aldehyde

primary alcohol

Product of reducing ketone

secondary alcohol

How to reduce carbonyls

React with LiAH4 in dry ether

- carbonyl gains 2 hydrogens

- room temp and pressure

Reaction of potassium cyanide and carbonyls

produces hydroxynitrile

- nucleophilic addition

- acidic conditions required for potassium cyanide to dissociate or just use HCN

Nucleophilic addition mechanism

1) cyanide ion attacks delta positive carbon

2) pair of electrons in double bond join O

3) hydrogen (from acid) bonds to the negative O

Potassium cyanide dangers

1) irritant

2) reacts with moisture in air to form toxic gas hydrogen cyanide

What is 2,4 DNPH

brady's reagent

Testing for a carbonyl group

2,4-DNPH AKA Brady's reagent

positive test = deep orange precipitate

- only ketones/aldehydes form ppt - other CO compounds don't (ester, carbonic acids)

How to identify carbonyl after Brady's

recrystallisation of precipitate to purify derivative of carbonyl then identified by comparing melting points with known values in database

How to add Brady's reagent to carbonyl

dissolve in conc sulfuric acid and methanol and then add to carbonyl

Reacting iodine with carbonyls (alkaline)

only carbonyls attached to a methyl group will react (ketones or ethanal)

- warm in alkaline conditions

- forms yellow crystalline precipitate with antiseptic smell of CHI3

are carbkxylic acids soluble ?

yes, small carboxylic acids only as they can hydrogen bond to water

What is a dimer?

where the carboxylic acid hydrogen bonds with each other

Why do carboxylic acids have higher boiling points than similar alcohols or aldehydes?

they form dimers

2 ways of making carboxylic acids

1) oxidation of alcohols and aldehydes

2) hydrolysis of nitriles

Oxidation of alcohol and aldehydes

form carboxylic acids

- acidified excess sodium dichromate under reflux

Hydrolysis of Nitriles

nitrile + 2h20 ---> carboxylic acid + NH3

- acidified under reflux

carboxylic acids reacting with LiAH4

reduction to alcohol

- LiAH4 in dry ether

carboxylic acids reacting with bases

salt + water + CO2

carboxylic acids with phosphorous (V) chloride

forms acyl chloride

- white misty fumes of HCl and can be a test for caboxylic acids

Forming esters with carboxylic acids

carboxylic acid + alcohol

- conc sulfuric acid catalyst under reflux

Naming Esters (IUPAC)

(alcohol) yl (carboxylic acid) anoate

acyl chloride functional group and suffix

-COCl

-anoyl chloride

Ester functional group and suffix

-COOC-

-oate

just remember COOCH

Uses of esters

solvents, perfumes and flavourings

2 ways to hydrolyse esters

1) with acidic conditions

2) with alkaline conditions

Hydrolysis of esters in acidic conditions

ester + water → carboxylic acid + alcohol

- reversible

- dilute acid under reflux

Hydrolysis of esters in alkaline conditions

ester + sodium hydroxide -> carboxylic salt + alcohol

- under reflux in alkaline conditions

- irreversible

Differences between acid and alkaline hydrolysis

1) the acid is a catalyst in acid hydrolysis but the alkali is a reagent in alkaline

2) reaction for acid is reversible

3) forms carboxylic salt in alkaline

How are polyesters formed?

from the condensation polymerisation of a diol and a dicarboxylic acid, with H2O given off in the reaction

- ester links

- carboxylic acid loses OH and alcohol loses H

Are acyl chlorides more reactive than carboxylic acids?

yes

Acyl chloride reacting with water

forms carbkxylic acid

- white steamy fumes of HCl

Acyl chloride reacting with alcohol

forms ester

- white steamy fumes of HCl

- better than using carboxylic acids as it's quicker and not reversible

Acyl chloride reacting with conc ammonia

forms primary amide

- white smoke of NH4Cl

Acyl chloride reacting with primary amines

forms secondary amine