Topic 18: Organic Chemistry III

2 classes of organic chemicals

aliphatic and aromatic

What is an aliphatic compound?

A compound that does not contain a benzene ring

What is an aromatic compound?

includes one or more benzene rings

2 structures of benzene

kekule and delocalised model

Features of kekules structure ?

1) alternative double and single bonds between carbons

2) differing bond lengths

Features of the delocalised model of benzene

1) the lone electrons in p orbitals combine to form delocalised ring

2) due to delocalised structure, all C-C bonds in molecule are same e.g. in length

What is the bond length in benzene?

all identical, between a sigma and a pi bond

Differences between delocalised structure and kekules

1) benzene is more stable than it should be

- enthalpy change of hydrogenation is much lower than if there were alternating double bonds

2) bond lengths are all the same whereas kekules suggests different lengths

Combustion of benzene

generally incomplete therefore a black smoky flame

Mechanism of alkenes and bromine

electrophilic addition

Does benzene undergo electrophilic addition and why?

no

- benzene is more stable than alkenes and therefore do not undergo electrophilic addition despite the high electron density of benzene as this would disrupt the stable ring of electrons

What mechanism do benzenes undergo?

electrophilic substitution

4 electrophilic substitution mechanisms

1) friedel crafts acylation

2) friedel crafts alkylation

3) halogenation

4) nitration

How do we form a very strong electrophile to react with benzene?

using a halogen carrier catalyst

Why does a benzene ring w side groups e.g. alkyl act more reactive than benzene?

methyl side group releases electrons into the delocalised system making it more attractive to electrophiles

Halogenation of benzene mechanism

Electrophilic substitution

- reagent of X2

- halogen carrier catalyst e.g.AlCl3

AlCl3 gains halogen to form an electrophile to attack

Nitration of benzene mechanism

Electrophilic substitution

- conc nitric acid and conc sulfuric acid which form NO2+ electrophile

- 60°C

- forms explosives

Nitration of benzene catalyst reaction

1) HNO3 ➕ 2H2SO4 🔜 NO2+ ➕ 2HSO4- ➕ H3O+

-Sulfuric + Nitric acid react to form nitronium ion, which then acts as an electrophile for the reaction

Nitration of benzene reforming catalyst

HSO4- + H+ -> H2SO4

Hydrogenation of Benzene

Reaction: benzene --> cyclohexane

Reagents: Hydrogen

Conditions: Nickel catalyst at 200°C and 30 atm

Type of reaction: Addition and reduction

Types of friedel crafts

alkylation and acylation

Friedel-Crafts Alkylation of Benzene

Change in functional group: benzene -> alkylbenzene

Reagents: chloroalkane in the presence of anhydrous aluminium chloride catalyst

Conditions: heat under reflux

Mechanism: Electrophilic Substitution

Friedel-Crafts Acylation of Benzene

Change in functional group: benzene->phenyl ketone

Reagents: acyl chloride in the presence of anhydrous aluminium chloride catalyst

Conditions: heat under reflux (50OC)

Mechanism: Electrophilic substitution

Effect of lone pairs on side groups of benzene

lone pairs join the delocalised system and cannot undergo general reaction e.g. nucleophilic substitution for halogenoalkanes

Properties of Phenols

1) OH directly attached to benzene ring

2) oxygen lone pairs in p orbital overlap and partially join delocalised ring

3) weakly acidic as CO bond strengthens and OH bond weakens

How to determine between carboxylic acid and phenol

add sodium carbonate

- phenols not strong enough to react and release CO2

Reaction of phenol with bromine

forms 2,4,6 tribromophenol and 3HBr

- white solid

- phenol lone pair is delocalised so electron density greater and bromine more polarised

Phenol v Benzene with bromine

1) phenol doesn't need halogen carrier catalyst

2) phenol undergoes multiple substitutions

3) bromine requires liquid bromine but phenol only requires bromine solution

4) phenol at room temp bromine needs reflux

Uses of phenols in industry

plastics and antiseptics

amine functional group

NH2, a secondary amine however hydrogens can be replaced with alkyl chains

primary amine

An amine where the nitrogen atom is attached to one alkyl chain

Secondary amine

An amine where the nitrogen atom is attached to two alkyl chains

tertiary amine

An amine in which the central nitrogen atom is connected to 3 carbon atoms.

Naming amines

list the names of the alkyl groups bonded to the N atom in alphabetical order in front of amine

-use prefixes to identify duplicate alkyl substituents

- use N-methylproplyamine to show that the amine group is between a methyl and propyl group

Amide functional group

C=O-N

What do amines smell of?

fish

Why can small amines dissolve in water ?

they can form hydrogen bonds with water

How are amines basic?

lone pair of electrons on the nitrogen atom allows for the forming of a dative bond to a hydrogen, accepting a proton and thus in water forms OH- ions

Why are primary aliphatic amines stronger bases than ammonia?

alkyl groups are electron releasing and push electrons towards the nitrogen atom and so make it a stronger base

Are secondary amines more basic than primary?

yes, more alkyl groups so therefore more electron density pushed on to nitrogen atom and being a stronger base

are tertiary amines more basic than primary and secondary?

no because they are less soluble in water due to alkyl chains

Why are aromatic amines not basic?

lone pair of electrons becomes delocalised into benzene ring and thus less able to accept protons

reacting amines with acids?

amines as bases react to form ammonium salts

- adding OH- will convert back to amine

CH3NH2 (aq) +HCl (aq) -> CH3NH3+Cl-(aq)

- no water like traditional neutralisation

Reacting amines with copper (II) ions

forms complex ions

- deep blue solution

- 4 CH3CH2NH2 + Cu(H2O)62+ -> [Cu(CH3CH2NH2)4(H2O)2] 2+ +4H2O

- the same ligand exchange reaction as ammonia

Reacting amines with halogenoalkanes

- nucleophilic substitution

- forms a amine of a higher order e.g. from a primary forms a secondary amine

- need excess amine

- same reacting as with ammonia

reacting primary amines with acyl chlorides

forms secondary amide and HCl

- HCl reacts further with amine to form ammonium salt

- room temp

- solid white product

- vigorous

How can we form primary aliphatic amines?

- from halogenoalkanes

- by reduction of nitriles

Forming primary aliphatic amines from halogenoalkanes

- reacting halogenoalkane with excess ethanolic ammonia

- forms amine

Disadvantage of making amines from halogenoalkanes

as the lone pair of electrons is still available on the N in the amine formed, the primary amine can react in the same nucleophilic way in a successive series of reactions forming secondary, tertiary amines and quaternary ammonium salts.

This is therefore not a good method for making a primary amine because of the further reactions. It would mean the desired product would have to be separated from the other products.

How to increase chance of only primary amine being formed

excess ammonia

how to form amines from nitriles ?

reduce using LiAlH4 in dry ether and dilute acid

- or using H2 with a nickel catalyst (cheaper)

What is a better way to form amines from halogenoalkanes?

1) add ethanolic KCN to halogenoalkane to form nitrile

2) reduce nitrile with LiAlH4

disadvantage if that method?

A disadvantage of this method is that it is a two step reaction that may therefore have a low yield. Also KCN is toxic.

forming aromatic amines

reducing aromatic nitro compounds by heating with tin and conc HCl under reflux

- forms ammonium salt so adding NaOH will form amine and water

Uses of aromatic amines

dyes and pharmaceuticals

How to form amides?

reacting amine with acyl chlorides

- forms secondary amide

- reacting amine with acyl chloride

- forms primary amide

How is a polyamide formed?

condensation polymerisation

- Two types of monomers react. One monomer contains two carboxylic acid groups and the other contains two amine groups. Water is also produced

2 ways to form polyester

- diol and dicarboxylic acid

- diol and diacyl dichloride

2 ways to form polyamide

- diamine and dicarboxylic acid

- diamine and diacyl dichloride

disadvantage of using dicarbkxylic acid

requires acid catalyst and only forms equilibrium mixture

disadvantage of using diacyl dichloride

produces hazardous HCl fumes

- but goes to completion and doesn't require catalyst

are polymers from condensation reactions biodegradable ?

yes they can be hydrolysed

intermolecular forces in polyamides

- hydrogen bonding between C=O and N-H

- permanent dipole between C=O and C-N

- london forces

intermolecular forces in polyesters

- permanent dipole due to C=O

- london forces

Do polyesters or polyamides tend to have higher boiling points and why?

polyamides as they can form hydrogen bonds

general structure of amino acid

central carbon atom bonded to amino group, a hydrogen atom, a carboxyl group, and an R group that differs for each amino acid

What is amphoteric?

a substance that can act as both an acid and a base

are amino acids amphoteric?

yes

in what state of amino acids exist?

zwitterions (dipolar)

What are zwitterions?

a molecule with both positive and negative ions

When are amino acids zwitterions

when the pH is at the isoelectric point and both carboxyl and amino groups ionised

what is the isoelectric point?

The pH at which there is no net charge

- dependent on the R group so different for each amino acid

zwitterion structure

- NH3+

- COO-

- ionic interaction

how are amino acids acidic or basic?

carboxylic acid group acid

amino group basic

How do amino acids form solids

ionic interaction between zwitterions explains the relatively high melting points of amino acids as opposed to the weaker hydrogen bonding that would occur in the no charge form.

amino acids in Acidic Solutions

- NH3+

- COOH

amino acids in basic solutions

- NH2

- COO-

do amino acids have optical isomerism?

yes

How do amino acids have optical isomerism?

they are chiral due to central carbon bonded to 4 different groups

- other than glycine

therefore they will rotate plane polarised light

shape of proteins

corkscrew

- due to hydrogen bonding

How to determined what amino acids form a protein

hydrolysis of protein and chromatography of amino acids

Condensation polymerisation of amino acids

forms polypeptide

Uses of grignard reagents

increasing carbon chain

What are Grignard reagents?

organomagnesium compounds

How to form a Grignard reagent?

reacting halogenoalkane with magnesium in dry ether

- R-X + Mg -> RMgX

Reacting grignard reagent with carbon dioxide steps

forms carboxylic acid

- step 1 in dry ether bubble CO2 into grignard reagent

- step 2 add dilute acid to protonate COO-

Reacting grignard reagents with aldehydes and ketones

forms alcohol

- step 1 in dry ether add aldehyde or ketone to grignard reagent

- step 2 add dilute HCl to protonate O-

Side products of using grignard reagents

Mg(OH)X

features of grignard reagents

nucleophiles and reducing agents

Why do we use reflux?

prevents organic vapours escaping and condenses them back into liquids

Why do we add anti bumping granules?

To get a more gentle boil by distributing the heat more evenly and providing a surface for small bubbles to form instead of large bubbles

why do we often use electric heaters instead of bunsens?

Electric heaters are often used to heat organic chemicals. This is because organic chemicals are normally highly flammable and could set on fire with a naked flame

- also allows us to control temperature

purification by washing

• Put the distillate of impure product into a separating funnel

• wash product by adding either

• sodium hydrogencarbonate solution , shaking and releasing the pressure from CO2 produced.

• Saturated sodium chloride solution

•Allow the layers to separate in the funnel, and then run and

discard the aqueous layer.

Removal of water by drying

- take impure product and add to round bottomed flask

- add anhydrous calcium chloride (dehydrating agent) which will remove any remaining aqueous substances

- invert flask and leave for 20 mins

- filter using filter paper to remove solid drying agent

what does the sodium chloride do?

helps separate organic layer from aqueous layer

What does the sodium hydrogen carbonate do

neutralise remaining acid

features of a drying agent

- insoluble in organic layer

- unreactive with organic layer

purifying an organic solid by recrystallisation process

1) dissolve impure compound in minimum volume of hot solvent

2) hot filtration of solution through filter paper

3) cool solution in ice

4) filter using vacuum filtration (Buchner funnel) to isolate crystals

5) wash crystals with distilled water

6) dry crystals between absorbent paper

Why do we use a minimum amount of hot solvent ?

to obtain a saturated solution to enable crystallisation on cooling

when is a solvent good?

An appropriate solvent is one which will dissolve both compound and impurities when hot and one in which the compound itself does not dissolve well when cold.