Edexcel a level chemistry topic 17

carboxyl group

COOH

physical properties of carboxylic acids

- able to form intermolecular H bonds due to hydroxyl group within their carboxyl group

- increasing boiling points with increasing carbon chain length

- solubility in water decreases with increasing carbon chain length

hydrogen bonding in carboxylic acids

- the effect of H bonding decreases as the alkyl chain increases in length because it is more likely to interrupt the H bonding that the carboxylic acid groups are able to make

- ethnic acid is an example of where the molecules are small enough that they are able to form dimerisation through H bonding

boiling points in carboxylic acids

- as the length of the carbon chain increases, the London forces between the non- polar hydrocarbon chain increase

- therefore, boiling temperature increases with increasing molecular mass, even though the effect of H bonding decreases

- the carboxyl group has 3 polar bonds, resulting in strong intermolecular forces and H bonding

- therefore, carboxylic acids have a higher boiling temp. compared to other organic compounds with a similar mass

solubility of carboxylic acids in water

- the effect of H bonding decreasing with increasing chain length affects solubility

- acids with 1-4 carbons are very soluble

- this is due to the highly polar C=O and O-H bonds, resulting in H bonds forming with water molecules

- as non- polar hydrocarbon chain becomes longer, solubility decreases

ways of preparing carboxylic acids

- oxidation of aldehydes

- oxidation of primary alcohols

- hydrolysis of esters

- hydrolysis of amides

- hydrolysis of nitriles

- hydrolysis of acyl chlorides

oxidation of primary alcohols and aldehydes

REAGENT: excess acidified potassium dichromate (VI), Cr2O7 2-/ H+

CONDITIONS: heat strongly under reflux

PRODUCT: carboxylic acid refluxing; separated by distillation

reduction of carboxylic acids

REAGENT: lithium tetrahydridoaluminate (LiAlH4)

CONDITIONS: dry ether

PRODUCT: primary alcohol and water

acid hydrolysis of esters

REAGENT: conc. sulfuric acid with excess water

CONDITIONS: heat

PRODUCT: alcohol and carboxylic acid

alkali hydrolysis of esters

REAGENT: sodium/ potassium hydroxide solution

CONDITIONS: heat

PRODUCT: alcohol and carboxylate salt (which can be converted to carboxylic acid by reacting with HCl)

esterification

REAGENT: carboxylic acid and alcohol with conc. sulfuric acid catalyst

CONDITIONS: heat

PRODUCT: ester

use of conc. H2SO4 in esterification reactions

- esterification is a reversible reaction which occurs at a slow rate

- H2SO4 is used as a drying agent

- H2SO4 increases yield and rate of reaction by dehydrating the system, causing the equilibrium to shift to the right and favour the forwards reaction

acyl chloride chemical properties

- attacked at the positive carbon centre by nucleophiles, such as water, alcohols, ammonia, amines

- undergo nucleophilic addition- elimination reactions

- more reactive than carboxylic acids and acid anhydrides

- very reactive with water

hydrolysis of acyl chlorides

REAGENT: acyl chloride + distilled water

CONDITIONS: RTP

PRODUCT: carboxylic acid and HCl

observations of hydrolysis of acyl chlorides

1. white misty fumes (HCl); damp universal indicator paper shows that the fumes are acidic because; fumes of ammonia mixed with HCl(g) produce a white smoke of ammonium chloride

2. fizzing and white solid

3. heat

preparation of acyl chlorides (halogenation)

REAGENT: carboxylic acid and phosphorus (V) chloride (PCl5)

CONDITIONS: RTP in anhydrous conditions

PRODUCT: acyl chloride, POCl3(l), HCl(g) (acyl chloride separated by fractional distillation)

observations of preparation of acyl chlorides

- vigorous reaction

- toxic HCl(g) forms

acid hydrolysis of nitriles

REAGENT: dilute HCl catalyst

CONDITIONS: heat under reflux

PRODUCT: carboxylic acid (separated by fractional distillation) and ammonium chloride

alkali hydrolysis of nitriles

REAGENT: sodium hydroxide

CONDITIONS: heat under reflux

PRODUCT: sodium carboxylate salt (reacts with HCl to produce carboxylic acid) and water

hydrolysis of aromatic nitriles

REAGENT: H2SO4 catalyst

CONDITIONS: heat under reflux

PRODUCT: carboxylic acid and ammonium sulphate

acyl chloride functional group

R-COCl

acyl chlorides and water

- react violently; reactions using acyl chlorides should be one in anhydrous conditions

- produces carboxylic acid and HCl

acyl chlorides and alcohols

- react readily

- form an ester and HCl gas

amines

- homologous series that contain the R-NH2 group

- react similarly with most carboxylic acid derivatives

- able to form N-substituted amides

ethanoyl chloride and methylamine (primary amine)

forms N-methylethanamide (secondary amide) and HCl

ethanoyl chloride and dimethylamine (secondary amine)

forms N, N-dimethylethanamide (tertiary amide)

ethanol chloride and tertiary amines

no reaction occurs

acyl chlorides and conc. ammonia

- forms an amide and HCl(g)

- a further reaction occurs, where basic NH3 reacts with acidic HCl to form NH4Cl

carbonyls

aldehydes and ketones

- contains C=O group

aldehyde group

RCHO

ketone group

RCOR'

formation of aldehyde

REAGENT: primary alcohol, potassium dichromate (VI)

CONDITIONS: heat gently and distill

PRODUCT: aldehyde

formation of ketone

REAGENT: secondary alcohol, potassium dichromate (VI)

CONDITIONS: heat under reflux

PRODUCT: ketone

prefix for ketone when it doesn't take priority

oxo-

physical properties of carbonyls

- C=O bond has highly electronegative O atom

- cannot intermolecular form H bonds due to lack of -OH group

- strongest intermolecular force that can be made is dipole- dipole forces

- only the smallest carbonyl compounds can form H bonds with water; this is due to the lone pair on the oxygen in the carbonyl group, which bonds with +ve H atoms in H2O molecules

- solubility decreases as carbonyl chain length increases

- BP increases with increasing chain length

why does solubility decrease as carbonyl size increases?

- longer alkyl chains disrupt the carbonyl compound from forming H bonds effectively

- regions of molecules are hydrophobic

order of BP of some organic molecules from highest to lowest

1. alkanes

2. ketones

3. aldehydes

4. alcohols

BP of ketones

- polar carbonyl group means that both London forces and dipole- dipole interactions are present

bond angle around a carbonyl group

120 degrees

Why are carbonyls susceptible to nucleophilic attack?

- carbonyls are highly susceptible to attack because of the pi bond electron clouds above and below the C=O bond

- the oxygen in the C=O bond is electronegative, so polarises the electron cloud and creates a partial charge on the carbon atom, therefore the carbon atom is now susceptible to electrophilic attack

Nucleophilic addition of carbons mechanism (HCN or KCN)

1. CN- acts as a nucleophile and attacks the slightly positive carbon

2. One of the C=O bonds breaks; a pair of electrons goes to the oxygen atom, which will now have a negative charge

3. A pair of electrons from the oxygen atom is used to form a bond with H+

4. Overall addition of CN to the molecule, therefore increasing carbon chain length

5. Hydroxynitrile produced can be optically active, but does not rotate plane polarised light

6. This is because the nucleophile can attack from above or below the molecule with equal probability to form a racemate

Why should acidified KCN be used instead of HCN in nucleophilic addition?

HCN is v. toxic and flammable and volatile

KCN is safer alternative

Reduction of carbonyls

REAGENT: sodium tetrahidridoborate (III) NaBH4 or Lithium tetrahydridoaluminate LiAlH4

CONDITIONS: dry ether

NUCLEOPHILE: H- (hydride ion)

MECHANISM: nucleophilic additions

Reduction of propanone

Oxidation of carbonyls

Aldehydes can oxidise to carboxylic acids but ketones cannot further oxidise

Tollens reagent

[Ag(NH3)2]+

- acts as an oxidising agent when heated gently with a carbonyl containing solution

Positive test for tollens

Silver mirror produced when aldehyde is present

Fehlings solution

- contains Cu2+ ions in a solution of sodium hydroxide

- solution oxidises aldehydes when heated with a sample

Positive Fehling's test

Solution changes from clear blue to opaque brick (due to copper(I) oxide being formed) in presence of aldehydes

Positive test for potassium dichromate

Orange to green in aldehydes

Why are carbonyl compounds difficult to distinguish?

They all have similar boiling points

Iodine in presence of alkali

REAGENTS: iodine + NaOH

CONDITIONS: warm very gently

PRODUCT: CHI3, a yellow crystalline precipitate with a distinct antiseptic smell

- only carbonyls with a methyl group next to the C=O bond undergo this, so mainly ketones and ethanal

2,4-dinitrophenylhydrazine

- C=O undergoes condensation reaction under acidic conditions with 2,4-DNPH

- when carbonyl is present, deep orange precipitate forms, which is purified by recrystallisation

- derivatives formed post- reaction gives clear and siting melting and boiling points

Structural isomerism

Same molecular formula, different structural formula

Chain isomerism

- isomers with different arrangement of carbon skeleton

- similar chemical properties

- slightly different physical properties

- more branches= lower BP

Position isomerism

-isomers with same carbon skeleton and same functional group

- functional group is in a different position

- similar chemical properties and slightly different physical properties

Functional group isomerism

- isomers with different functional groups

- exhibit different chemical and physical properties

Stereoisomerism

-Same molecular formula but atoms occupy different positions in space

- relies on existence of at least 2 groups

- on two different carbons

- bonded together by a C=C double bond, which restricts rotational movement

Geometrical isomerism

- occurs due to the restricted rotation of the C=C double bonds

- E-Z isomerism

- cis- trans isomerism

Optical isomerism

- Occurs when molecules have a chiral centre

- 2 non- superimposable mirror images

E/Z isomerism

- groups around C=C bond are prioritised by their atomic numbers

- E( entgegen): higher priority group on opposite sides of C=C bond

- Z( zusammen): higher priority group on same side of C=C bond

Cis-trans isomerism

- based on longest carbon chain present

Identifying chiral centres

- optical isomerism arises due to orientation and positional difference of functional groups in 3D space in a molecule

- chiral centre causes this

Chiral centre

Carbon atoms that are found in the molecule that have different/ unique groups of molecules bonded to it, arranged in a tetrahedral structure

Enantiomers/ conditions for optical isomerism

- mirror images of each other

- non- superimposable

How to distinguish enantiomers

- they should rotate plane polarised light in two different directions

Polorimeter

1. Light source produces light vibrating in all directions

2. Polarising filter only allows through light vibrating in one direction

3. Plane polarised light passes through samples

4. If sample is optically active, it rotates the plane polarised light

5. Analysing filter is turned so that light reaches a maximum

6. Direction of rotation is measured coming towards the observer

Dextrorotatory

Rotates plane-polarized light to the right

Laevorotatory

rotates plane polarised light to the left

Racemate

- 50/ 50 mixture of the two enantiomers

- opposite optical effects of each isomer cancel each other out

Formation of enantiomers

- carbonyl compounds undergo nucleophilic addition

- if there are two different groups attached to the C=O bond, the possibility of forming optical isomers arises

E.g. is CN- attacks carbonyl from above, one optical isomer is formed, and another is formed if it attacks from below the planar molecule

- two non- superimposable mirror images produced

- race mix mixture because there is a 50/ 50 chance of attack from either above or below the plane

Thalidomide

- used to treat anxiety/ morning sickness is pregnant women

- many babies born with deformities/ missing limbs

- only one enantiomer was safe

racemic mixture

50/50 composition of each enantiomer

- there would be no rotation of plane polarised light because they cancel each other out