Reactions of Organic Compounds

The Alkanes homologous series with the general formula CnH2n+2.

have trends in physical properties e.g. density and m.p. and b.p. all increase with Mr. all undergo similar chemical reactions.

SATURATED HYDROCARBONS. i.e. they contain only single C to C bonds and are made up of C and H atoms only.

Alkanes are obtained from crude oil by fractional distillation.They are mainly used as fuels.The large Mr alkanes do not ignite easily so there is little demand for them as fuels so they are CRACKED to make smaller more useful alkanes and alkenes.Apart from combustion alkanes undergo few chemical reactions. This is for two main reasons:

• The bonds in alkanes are relatively strong.• The bonds have a relatively low polarity as the

electronegativity of C and H is similar.As a consequence alkanes can be used as lubricating oils, although they do degrade over time.

1. Similar for all alkanes2. Not very reactiveReasons:3. C-H and C-C covalent bonds essentially

non-polar, unlikely to attract polar molecules or ions.

4. Strong C-C and C-H bonds require a lot of energy to break.

Chemical Properties of Alkanes

Smaller alkanes burn easily in air when ignited If the combustion is complete, products formed are

CO2 and H2O.

CH4 + 2O2 CO2 + H2O H = -890kJ/mol

Reaction is exothermic - a result of the high relative strength of the C=O in CO2 and O-H in H2O

molecules. The large amount of heat energy released in making these bonds means the reaction is strongly exothermic.

The combustion of Alkanes

Write equations for the complete combustion of butane and octane.

If the combustion is incomplete, products formed are CO, C (soot) and H2O.

2C H4 + 3O2 2CO + 4H2O CH4 + O2 C + 2H2O

What problems do the gases released on combustion of alkanes cause?

In the presence of ultraviolet light, methane can combine with chlorine to give a mixture of products (chloroalkanes)

Light energy is used to start the reaction (by providing the energy to break the covalent bond between the chlorine atoms in Cl2.

The chlorine atom(radical) produced then reacts with alkane by substituting the hydrogen atom.

Reactions of Alkanes with halogens – Substitution reactions

Free radical = species with an unpaired electron.Free radicals are formed by homolytic fission of bonds. In homolytic fission one electron from the shared pair goes to each atom.So

ClCl

ClCl +

or Cl2 2Cl.

unpaired electron

Heterolytic fission of Cl – Cl would result in the formation of Cl+ and Cl-.There are three steps in the mechanism: initiation, propagation and termination

CH4 + Cl2 CH3Cl + HCl

Overall reaction equation

Conditions

ultra violet light (breaks weakest bond)

excess methane to reduce further substitution

i.e. homolytic breaking of covalent bonds

Free radical substitution

chlorination of methane

CH4 + Cl CH3 + HCl

Cl2 Cl + Cl

CH3 + Cl2 CH3Cl + Cl

CH3ClCH3 + Cl

initiation step

propagation steps

termination step

UV Light

CH3CH3CH3 + CH3

Free radical substitution mechanism

Also get reverse of initiation step occurring as a termination step.

Cl2 Cl + Cl

CH3Cl + Cl2 CH2Cl2 + HCl

Overall reaction equations

Conditions

CH2Cl2 + Cl2 CHCl3 + HCl

CHCl3 + Cl2 CCl4 + HCl

ultra-violet light

excess chlorine

Further free radical substitutions

Details of free radical substitution Page 435

general formula CnH2n. unsaturated hydrocarbons containing C =

C.

Much more reactive than alkanes.

Industrial importance of alkenes:

1. Making polymers (plastics)

2. Hydrogenation of vegetable oils to make margarine

3. Hydration of ethene to make ethanol.

The Alkenes

When naming alkenes have to include position of double bond, for example:

CH3CH=CHCH3 is but - 2 - ene andCH3CH2CH=CH2 is but -1- ene

Alkenes undergo ADDITION reactions. Two substances combine to form one new substance. Unsaturated molecules are converted to saturated molecules.

More than 1 C=C bond

Page 440

1. Similar for all alkenes2. More reactive than alkanes

Reasons:3. C=C covalent bonds (4 electrons) represents

a region of high electron density, therefore attracts electrophile ( + ion / + end of dipole )

4. Ä bond in C=C can break easily – allow other atoms to join to the carbon atoms resulting in an addition reaction.

Chemical Properties of Alkenes

Alkenes burn in an excess air to form CO2 and H2O.

C2H4 + 3O2 2CO2 +2H2O H = -1322kJ/mol

Reaction is exothermic.

If the combustion is incomplete, products formed are CO, C (soot) and H2O. C2H4 + O2 2C + 2H2O C2H4 + 2O2 2CO + 2H2O

More soot (C) is produced compared to the corresponding alkane, due to higher percentage by mass of carbon.

The combustion of Alkenes

How can you tell whether the reaction is complete or incomplete?

R : alkyl group or H

Addition Reactions

C =CR

R R

R+ X-Y R – C – C – R

X Y

R R

Hydrogenation (with hydrogen) Addition of hydrogen (hydrogenation)

Alkenes react with hydrogen in the presence of a nickel catalyst at 150 °C to form an alkane. e.g.

H – C – C – H C =CH

H H

H+ H2

H H

H H

C2H4 + H2 C2H6

ethene ethane

Ni / 1500C

Hydrogenation is used in margarine industry to convert oils containing unsaturated hyrocarbon chains into saturated compounds with higher melting points.

Margarine is a solid at room temperature

Hydrogenation

Addition of halogens (halogenation) Halogens react with alkenes at room

temperature and pressure in a non-polar solvent to form a dihalogenoalkane.

Halogenation (with halogens,X2)

C =CH

H H

H+ Cl2 H – C – C – H

Cl Cl

H H

C2H4 + Cl2 C2H4Cl2

ethene 1,2-dichloroethaneWrite the equation when propene reacts with chlorine.

C =CH

H H

H+ HBr H – C – C – H

H Br

H H

C2H4 + HBr CH3CH2Br

ethene bromoethane

With hydrogen halides (H-X) Alkenes react with hydrogen halides (HCl,

HBr etc.) to solvent to form a dihalogenoalkane.

The reaction occurs at room temperature and pressure. e.g.

C =CH

H H

H+ H2O

OH

H – C – C – H

H

H H

Hydration (with water) Hydration reaction can be done in 2 ways:

addition of concentrated sulfuric acid at room temperature and warm with water to produce an alcohol.

Conc. H2SO4

heat

ethanol

Bromine water is used as a test for unsaturation.In the presence of an alkene, bromine water turns from red brown to colourless. Alkanes do not react with bromine water.

C =C – C – C –

Br Br

+ Br2(aq)

colorless orange colorless

Test for Unsaturation – bromine water test for alkenes

heat

monomer polymer

A large number of monomers are joined together into a polymer.

Addition Polymerisation

C =CR

R R

R

–C – C–

R R

R R

n

n

ethene poly(ethene)

Poly(ethene)

C =CH

H H

H

–C – C–

H H

H H

n

n

2000C, 2000 atm

O2 / peroxide

ethene poly(chloroethene)

Poly(vinylchloride)

C =CH

H H

Cl

–C – C–

H Cl

H H

n

nrepeating unit

ethene poly(ethene)

Poly(propene)

C =CH

H H

CH3

C – C

H CH3

H H

n

n

repeating unit

monomer repeating unit polymer typical uses

CH2=CH2 - CH2 – CH2 - poly(ethene)polythene

film, bags

poly(propene)polypropylene

Moulded plastic, fibres

poly(phenylethene)polystyrene

packaging,insulation

CH2=CHCH3 - CH2 – CH -

CH3

CH2=CHC6H5 - CH2 – CH -

C6H5

monomer repeating unit

polymer typical uses

pipes, flooringCH2=CHCl - CH2 – CH -

Cl

poly(chloroethene)polyvinylchloridePVC

Poly(tetrafluoroethene)PTFE

CF2=CF2 - CF2 – CF2 - Non-stick coating (Teflon)

Practice questions Page 446

1. To make margarine2. To make alcohols, used as antifreeze and

solvents3. To make plastic (polymers) like

poly(ethene), PVC4. Used in agriculture (in low concentration)

to help hasten the ripening of fruits like bananas.

Uses of Alkenes

2CH3OH(l) + 3O2(g) 2CO2(g) + 4H2O(l) ΔH = -726kJ/mol

C2H5OH(l) + 3O2(g) 2CO2(g) + 3H2O(l) ΔH = -1371kJ/mol

In countries such as Brazil, ethanol is mixed with petrol and used to power cars. Ethanol is less efficient as a fuel than petrol as it is already partially oxidised but does make the country less reliant on supply of petrol. As it can be produced by fermentation of sugar beet, many consider ethanol a carbon neutral fuel.

The combustion of Alcohols

OHH

H

C

H

H

C

H

+ H2OO

H

H

C

H

CH

ethanol ethanal

The oxidation reactions of Alcohols Primary alcohols are oxidised first to

aldehydes. A suitable oxidising agent is acidified potassium dichromate(VI)

Cr2O72-

/H+

heat

An aldehyde still has one hydrogen atom attached to the carbonyl carbon, so it can be oxidised one step further to a carboxylic acid.

OH

H

C

H

CH

OH

H

C

H

COH

ethanal ethanoic acid

Cr2O72-

/H+

10 alcohol aldehyde carboxylic acid Cr2O7

2- /H+

Cr2O72-

/H+

Inductive reasoning – applicable to another 10 alcohol Page 450

heat

heat heat

In practice, a primary alcohol such as ethanol is dripped into a warm solution of acidified potassium dichromate(VI).

The aldehyde, ethanal, is formed and immediately distils off, thereby preventing further oxidation to ethanoic acid, because the boiling point of ethanal

(23 °C) is much lower than that of either the original alcohol ethanol (78 °C) or of ethanoic acid (118 °C). Both the alcohol and the acid have higher boiling points because of hydrogen bonding.

If oxidation of ethanol to ethanoic

acid is required, the reagents must

be heated together under reflux to

prevent escape of the aldehyde before

it can be oxidised further.

H

H

C

H OH

H

C

H

H

C H

propan-2-ol

H

H

C

H O

H

C

H

C H

propanone

Secondary alcohols are oxidised to ketones. These have no hydrogen atoms attached to the carbonyl carbon and so cannot easily be oxidised further.

Tertiary alcohols are resistent to oxidation

Cr2O72-

/H+

heat

Distinguishing between 1°, 2° and 3° alcohols

When orange acidified potassium dichromate(VI) acts as an oxidising agent, it is reduced to green chromium(III) ions.

1° and 2° alcohols both turn acidified dichromate(VI) solution from orange to green when they are oxidised, and this colour change can be used to distinguish them from 3° alcohols.

3° alcohols are not oxidised by acidified dichromate(VI) ions, so they have no effect on its colour, which remains orange. Practice Page 452

An aldehyde can be oxidised by heating with acidified potassium dichromate(IV) to carboxylic acid.

OH

H

C

H

CH

OH

H

C

H

COH

ethanal ethanoic acid

Cr2O72-

/H+

heat

Reactions of aldehydes and ketones

The + carbon atom is susceptible to attack by NUCLEOPHILES.

A nucleophile is a species with a lone pair of electrons. E.g. OH-, NH3, CN-.

When attack by a nucleophile occurs, the carbon – halogen bond breaks releasing a halide ion.

A suitable nucleophile for experimentation is OH- from an aqueous solution of an alkali such as sodium hydroxide.

Nucleophilic substitution reactions

CH3CH2CH2X + OH- CH3CH2CH2OH + X-

OH has replaced the X so overall we have NUCLEOPHILIC SUBSTITUTION

X = Cl, Br or I.

H

Br

C

H H

H

C

H

H

C H H

OH

C

H H

H

C

H

H

C HNaOH(aq)

heat

1-bromopropane Propan-1-ol

A nucleophile is a molecule / negatively charged ion with a lone pair of electrons which is attacted to a more positively charged region in a molecule and donates a lone pair of electrons to form a covalent bond.

Mechanisms for nucleophilic substitution

SN1 = unimolecular nucleophilic substitution (only one species in the slow step of the mechanism, rate determining step)

SN2 = bimolecular nucleophilic substitution (two species in the slow step of the mechanism, rate determining step)

Use of curly arrows:

Curly arrows are used in reaction mechanisms to show the movement of electron pairs.

to forma lone pairof electrons

X

eithera bond pairof electrons

or a lone pairof electrons

eithernext to an atom

TAILS come from

HEADS point

to form a bond pair of electrons

orat an atom

X

-OH

CH3

H

OHC

H

Br-

CH3

H

BrC

H

CCH3

H

H

+ +

CCH3

H

H

+

slow

fast

SN1

Intermediate carbocation

Heterolytic fission of C – Br bond

Page 461

-OH

HO

H

CH3C

H

Br-

BrC

CH3

H

H

+

C

H

CH3 H

HO Br

-

SN2

Transition state

Page 460

Which is best? SN1 or SN2?

For primary halogenoalkanes – SN2For tertiary halogenoalkanes – SN13° halogenoalkanes cannot undergo the SN2 mechanism as 5 bulky groups would not fit around the C in the transition state - steric hindrance.1° halogenoalknes are less likely to undergo SN1 as this would involve the formation of a primary carbocation as an intermediate. Alkyl groups push electron density to the C atom they are attached to (positive inductive effect) which stabilises the positive charge. More alkyl groups mean a more stable carbocation.

2° halogenoalkanes react via a mixture of SN1 and SN2. The mechanism predominating depends upon the nature of the alkyl groups and the nature of the solvent.

Reaction pathways

alkane

halogenoalkane

alcohol

ketone

dihalogenoalkane

alkene

aldehyde

poly(alkene)

carboxylic acid

trihalogenoalkanetetrahalogenoalkane

M1

M2

M1 and M2 : You should know the mechanisms for these reactions

The flow chart above enables you to convert 2-bromobutane into butanoneusing a two-step synthetic route

2-bromobutane butan-2-ol butanone reflux with NaOH reflux with H+/Cr2O7

Page 482-486

Devise two-step syntheses of the following products from the starting material. Include any experimental conditions.

(a) Ethanoic acid from ethene(b) Butan-1-ol from butane(c) Propanal from 1-bromopropane

Example