Bonding in methane, ethane

and ethene and bonds

AS Chemistry

Learning Objectives

Candidates should be able to:

•describe covalent bonding in terms of orbital overlap, giving and bonds.

•explain the shape of, and bond angles in, ethane and ethene molecules in terms of and bonds.

Starter activity

Alkenes

pent-2-ene CH3CH=CHCH2CH3

hex-3-ene CH3CH2CH=CHCH3

2,3-dimethylpent-2-ene

cyclopenta-1,3-diene

3-ethylhept-1-ene CH2=CHCH2CH(CH2CH3)CH2CH2CH3

Hybridisation of orbitals

The electronic configuration of a carbon atom is 1s22s22p2

1 1s

2

2s

2p

HYBRIDISATION OF ORBITALS

If you provide a bit of energy you can promote (lift) one of the s electrons into a p orbital. The configuration is now 1s22s12p3

1 1s

2

2s

2p

The extra energy released when the bonds form more than compensates for the initial input.

Hybridisation of orbitals in alkanesThe four orbitals (an s and three p’s) combine or HYBRIDISE to give four new orbitals. All four orbitals are equivalent.

Because one s and three p orbitals are used, it is called sp3 hybridisation.

2s22p2 2s12p3 4 x sp3

sp3 orbitals

In ALKANES, the four sp3 orbitals repel each other into a tetrahedral arrangement.

Hybridisation of orbitals in alkanes

Bonding in methane

Bonding in ethane

Bonding in ethene

Alternatively, only three orbitals (an s and two p’s) combine or HYBRIDISE to give three new orbitals. All three orbitals are equivalent. The remaining 2p orbital is unchanged.

2s22p2 2s12p3 3 x sp2 2p

sp2 hybrids

What about ethene?

- bonds

Geometric Isomerism

AS Chemistry

Learning Objectives

Candidates should be able to:

describe cis-trans isomerism in alkenes, and explain its origin in terms of restricted rotation due to the presence of π bonds.

deduce the possible isomers for an organic molecule of known molecular formula.

identify cis-trans isomerism in a molecule of given structural formula.

Starter activity

ISOMERISM

STRUCTURAL ISOMERISM STEREOISOMERISM

GEOMETRIC ISOMERISM OPTICAL ISOMERISM

What is stereoisomerism?

In stereoisomerism, the atoms making up the isomers are joined up in the same order, but still manage to have a different arrangement in space

Geometric Isomerism?

GEOMETRIC ISOMERISMRESTRICTED ROTATION OF C=C BONDS

Single covalent bonds can easily rotate. What appears to be a different structure in an alkane is not. Due to the way structures are written out, they are the same.

ALL THESE STRUCTURES ARE THE SAME BECAUSE C-C BONDS HAVE ‘FREE’ ROTATION

Animation doesn’t work in old versions of Powerpoint

Geometric Isomerism?

Geometric isomers of but-2-ene

X

Geometric Isomerism?

GEOMETRIC ISOMERISMHow to tell if it exists

Two different atoms/groups attached

Two different atoms/groups attached

Two similar atoms/groups attached

Two similar atoms/groups attached

Two similar atoms/groups attached

Two different atoms/groups attached

Two different atoms/groups attached

Two different atoms/groups attached

GEOMETRICAL ISOMERISM

GEOMETRICAL ISOMERISM

Once you get two similar atoms/groups attached to one end of a C=C, you cannot have geometrical isomerism

GEOMETRIC ISOMERISMIsomerism in butene

There are 3 structural isomers of C4H8 that are alkenes*. Of these ONLY ONE exhibits geometrical isomerism.

BUT-1-ENE 2-METHYLPROPENE

trans BUT-2-ENEcis BUT-2-ENE

* YOU CAN GET ALKANES WITH FORMULA C4H8 IF THE CARBON ATOMS ARE IN A RING

Summary

To get geometric isomers you must have:

restricted rotation (involving a carbon-carbon double bond for A-level purposes);

two different groups on the left-hand end of the bond and two different groups on the right-hand end. It doesn't matter whether the left-hand groups are the same as the right-hand ones or not.

The effect of geometric isomerism on physical properties

isomermelting point (°C)

boiling point (°C)

cis -80 60

trans -50 48

You will notice that:the trans isomer has the higher melting point;the cis isomer has the higher boiling point.

Why is the boiling point of the cis isomers higher?

The difference between the two is that the cis isomer is a polar molecule whereas the trans isomer is non-polar.

Why is the melting point of the cis isomers lower?

In order for the intermolecular forces to work well, the molecules must be able to pack together efficiently in the solid.

Trans isomers pack better than cis isomers. The "U" shape of the cis isomer doesn't pack as well as the straighter shape of the trans isomer.

Optical Isomerism

AS Chemistry

Learning Objectives

Candidates should be able to:

explain what is meant by a chiral centre and that such a centre gives rise to optical isomerism.

deduce the possible isomers for an organic molecule of known molecular formula.

identify chiral centres in a molecule of given structural formula.

 

Starter activity

Optical isomerism

When four different atoms or groups are attached to a carbon atom, the molecules can exist in two isomeric forms known as optical isomers. These are non-superimposable mirror images.

Chiral centre

Chiral molecule

Optical IsomerismWhat is a non-superimposable mirror image?

Animation doesn’t work in old versions of Powerpoint

Optical isomerism

Amino acids (the building blocks of proteins) are optically active. They affect plane polarised light differently.

Butan-2-ol

Optical IsomerismThe polarimeter

If the light appears to have turned to the right turned to the left DEXTROROTATORY LAEVOROTATORY

A Light source produces light vibrating in all directionsB Polarising filter only allows through light vibrating in one directionC Plane polarised light passes through sampleD If substance is optically active it rotates the plane polarised lightE Analysing filter is turned so that light reaches a maximumF Direction of rotation is measured coming towards the observer

A B

C DE

F

Enantiomers – how do they differ?

Usually have the same chemical and physical properties – but behave differently in presence of other chiral compounds.

Enantiomers – how do they differ?

TYPES OF ISOMERISM

Occurs due to the restricted rotation of C=C double bonds... two forms - CIS and TRANS

STRUCTURAL ISOMERISM

STEREOISOMERISM

GEOMETRICAL ISOMERISM

OPTICAL ISOMERISM

CHAIN ISOMERISM

Same molecular formula but different structural formulae

Occurs when molecules have a chiral centre. Get two non-superimposable mirror images.

Same molecular formula but atoms occupy different positions in space.

POSITION ISOMERISM

FUNCTIONAL GROUP ISOMERISM

Electrophilic Addition to

Alkenes

AS Chemistry

Learning Objectives

Candidates should be able to:

• describe the mechanism of electrophilic addition in alkenes, using bromine/ethene as an example.

• describe the chemistry of alkenes as exemplified, where relevant, by the following reactions of ethene: addition of hydrogen, steam, hydrogen halides and halogens.

Starter activity

CH2=CH2 + Br2 CH2BrCH2Br

Electrophilic addition

Electrophilic addition

CH2=CH2 + Br2 CH2BrCH2Br

bromine with ethene

hydrogen bromide with ethene

CH2=CH2 + HBr CH3CH2Brbromoethane

1,2-dibromoethane

Br

Br

Br

Br

Electrophilic addition mechanism

H

H H

H

CC

+

-

H

H H

HCC

Br+

Br-

carbocation

H

H H

HCC

Br Br1,2-dibromoethane

bromine with ethene

Electrophilic addition mechanism

H

H H

H

CC

H

H H

HCC

H+

carbocation

H

H H

HCC

Br H

bromoethane

hydrogen bromide with ethene

-

+

Br

H

Br-

Electron flow during electrophilic addition

EQUATION TEMPERATUR

E(OC)

PRESSURE CATALYST PHASE NOTES

hydrogen CH2=CH2 + H2 → CH3CH3 ~150

Finely divided nickel on support material

GasNever carried out industrially. Analogous reaction used to produce some margarines from oils (see later).

steam CH2=CH2 + H2O→ CH3CH2OH

330 6MPaPhosphoric (V) acid (H3PO4) adsorbed onto the surface of silica.

GasMajor industrial process for the manufacture of ethanol.

hydrogen halides

(e.g. HBr)

CH2=CH2 + HBr → CH3CH2Br

Room temperature

Aqueous solution

Reactivity increases from HF to HI.

halogens CH2=CH2 +Br2→ CH2BrCH2Br

Room temperature

Liquid bromine or solution (both aqueous and non-polar solvent.

Chlorine and iodine produce similar addition products. Fluorine is too powerful an oxidizing agent.Addition reactions of

alkenes

Addition to unsymmetrical alkenesElectrophilic addition to propene

2-bromopropane

1-bromopropane

In the electrophilic addition to alkenes the major product is formed via the more stable carbocation (carbonium ion)

least stable most stable

methyl < primary (1°) < secondary (2°) < tertiary (3°)

Addition to unsymmetrical alkenes

PATH A

PATH B

MAJOR PRODUCT

PRIMARYCARBOCATION

SECONDARYCARBOCATION

MINOR PRODUCT

Addition to unsymmetrical alkenes

Polymerisation

AS Chemistry

Learning Objectives

Candidates should be able to:

describe the chemistry of alkenes including polymerisation.

describe the characteristics of addition polymerisation as exemplified by poly(ethene) and PVC.

Recognize the difficulty of the disposal of poly(alkene)s, i.e. non-biodegradability and harmful combustion products.

Starter activity

Poly(ethene)

Temperature: about 200°C

Pressure: about 2000 atmospheres

Initiator: often a small amount of oxygen as an impurity

Conditions

Free radical addition

Initiation

Propagation

Termination

Propagation

LDPE or HDPE

LDPE or HDPE

Freezer bags, water pipes, wire and cable insulation, extrusion coating

Sandwich bags, cling wrap, car covers, squeeze bottles, liners for tanks and ponds, moisture barriers in construction

Polymerisation of alkenes

ETHENE POLY(ETHENE)

TETRAFLUOROETHENE

POLY(TETRAFLUOROETHENE)

PTFE “Teflon”

PROPENE POLY(PROPENE)

CHLOROETHENEPOLY(CHLOROETHENE)

POLYVINYLCHLORIDE PVC

Method CommentsLandfill Emissions to the atmosphere and

water; vermin; unsightly. Can make use of old quarries.

Incineration Saves on landfill sites and produces energy. May also release toxic and greenhouse gases.

Recycling high cost of collection and re-processing.

Feedstock recycling

Use the waste for the production of useful organic compounds. New technology can convert waste into hydrocarbons which can then be turned back into polymers.

Disposal of polymers

Oxidation of alkenes

AS Chemistry

Learning Objectives

Candidates should be able to describe the oxidation of alkenes by:

cold, dilute, acidified manganate(VII) ions to form the diol, and

hot, concentrated, acidified manganate(VII) ions leading to the rupture of the carbon-to-carbon double bond in order to determine the position of alkene linkages in larger molecules.

Starter activity

Oxidation of alkenes

In the presence of dilute (acidified or alkaline) potassium manganate (VII).

•Alkenes react readily at room temperature (i.e. in the cold).

•The purple colour disappears and a diol is formed.

CH2=CH2 + H2O + [O] HOCH2CH2OHethane – 1,2-diol

Oxidation of alkenes

Fragment Product

=CH2CO2

R-CH=

Aldehyde → carboxylic acid

R2C=Ketone

In the presence of a hot, concentrated solution of acidified potassium manganate (VII), any diol formed is split into two fragments which are oxidized further to carbon dioxide, a ketone or a carboxylic acid.

Oxidation of alkenes1. CH2=CH2

2. CH3CH=CH2

3. (CH3)2C=CH2

2 products – both contain ketone

2 products – one contains 2 ketone groups and one contains 2 acid groups.

1 product only

Halogenoalkanes

AS Chemistry

Learning Objectives

Candidates should be able to recall the chemistry of halogenoalkanes as exemplified by the following nucleophilic substitution reactions of bromoethane:

hydrolysis;

formation of nitriles;

formation of primary amines by reaction with ammonia.

Starter activity

a. CHCl3 trichloromethaneb. CH3CHClCH3 2-chloropropanec. CF3CCl3 1,1,1-trichloro-2,2,2-

trifluoroethane

Naming Halogenoalkanes

F

F

Cl

Cl

F Cl

Physical Propertiesa. 1-chloropropane is polar and has permanent

dipole-dipole intermolecular forces that are stronger than the temporary dipole-induced dipole forces in non-polar butane.

b. 1-chloropropane is polar and has permanent dipole-dipole intermolecular forces that are stronger than the temporary dipole-induced dipole forces in non-polar butane.

Nucleophilic substitution negotiate clever

alp or

cadet tart

eat given

enticed if

chenille soup

had lie

stubs tuition

electronegative

polar

attracted

negative

deficient

nucleophiles

halide

substitution

Nucleophilic substitution

This is known as an SN2 reaction. S stands for substitution,

N for nucleophilic, and

2 because the initial stage of the reaction involves two species.

Nucleophilic substitution - mechanism

ANIMATION SHOWING THE SN2 MECHANISM

Attack by nucleophile is to the back of the molecule – away from the negatively charged halogen atom.

Rate of reaction

You may expect the fluoroalkane to react more quickly as the C-F bond is the most polar and therefore more susceptible to attack by nucleophiles. However, the C-F bond is the strongest. A nucleophile may be more attracted more strongly to the carbon atom but, unless it forms a stronger bond to carbon, it will not displace the halogen.

Actually the reaction with the iodoalkane is the most rapid. This suggests that the strength of the C-X bond is more important than its polarity. Note that the C-I bond is not polar. However, it is easily polarisable.

Halogen F Cl Br IElectronegativity 4.0 3.0 2.8 2.5

Bond strength (C-X) kJ mol-1

484 338 276 238

Experiment

Water is a poor nucleophile but it can slowly displace halide ions C2H5Br(l) + H2O(l) C2H5OH(l) + H+

(aq) + Br¯(aq)

If aqueous silver nitrate is shaken with a halogenoalkane (they are immiscible) the displaced halide combines with a silver ion to form a precipitate of a silver halide. The weaker the C-X bond the quicker the precipitate appears.

Measuring the rate of reaction

hydroxide ion with bromoethane

ethanolCH3CH2Br + OH- CH3CH2OH + Br-

(aqueous)

Nucleophilic substitution

Water with bromoethane

ethanolCH3CH2Br + H2O CH3CH2OH + HBr(aqueous)

This is a slower reaction – water is not such a good nucleophile.

warm

warm

+ -CH3

H

BrC

H

-OH

CH3

H

OHC

H Br-

hydroxide ion with bromoethane

Nucleophilic substitution mechanism

ethanol

water with bromoethane

Nucleophilic substitution mechanism

ethanol

+ -CH3

H

BrC

H

Br-

H

CH3

H

OHC

H

+

CH3

H

OHC

HHBr

H2O

propanenitrile

CH3CH2Br + CN-(ethanol) CH3CH2CN + Br-

cyanide ion with bromoethane

ammonia with bromoethane

CH3CH2Br + NH3(ethanol) CH3CH2NH22 + NH4+Br-

Nucleophilic substitution

aminoethane

CH3CH2Br + NH3(ethanol)CH3CH2NH2 + HBr

reflux

Heat /

pressure

Heat /

pressure

+ -CH3

H

BrC

H

CN-

CH3

H

CNC

H Br-

cyanide ion with bromoethane

Nucleophilic substitution mechanism

propanenitrile

ammonia with bromoethane

Nucleophilic substitution mechanism

aminoethane

+ -CH3

H

BrC

H

Br-

H

CH3

H

NH2C

H

+

CH3

H

NH2C

H

NH3

H NH3+Br -

NH3

Past paper question

Cl2U.V. /sunlight

Ethanolic KCNreflux

Br2

U.V. /sunlight

Substitution vs. Elimination

AS Chemistry

Learning Objectives

Candidates should be able to:

recall the chemistry of halogenoalkanes as exemplified by the elimination of hydrogen bromide from 2-bromopropane.

describe the mechanism of nucleophilic substitution (by both SN1 and SN2 mechanisms) in halogenoalkanes.

Starter activity

Type of halogenoalkan

e

Position of halogeno-

group

Example

primary at end of chain: bromoethane

secondary in middle of chain: 2-bromopropane

tertiary attached to a carbon atom which carries no H atoms:

2-bromo-2-methylpropane

SN1 – tertiary halogenoalkanes

Nucleophilic attack at the back of the molecule is hindered by bulky CH3 groups. Tertiary carbocation is stabilised by electron donating effect of CH3 groups.

SN1 or SN2 ?

Halogenoalkane Mechanism

Primary SN2

Secondary SN1 and SN2

Tertiary SN1

Elimination

You need to be aware that the hydroxide ion can act as a strong base as well as a nucleophile.

An alternative reaction can take place in which HBr is removed and an alkene is formed. This is known as elimination.

CH3CH2Br + NaOH CH2=CH2 + NaBr + H2O

Elimination of HBr from 2-bromopropane

CH3

H H

H

CC

OH-

CH3

H H

HCC

Br H

propene

H OHBr -

CH3CHBrCH3 + OH- CH3CH=CH2 + H2O + Br-

(in ethanol)

acting as a base

Elimination of HX from haloalkanes

92

elimination

+ OH-

RCH=CH2 + H2O + X-

(ethanol)

nucleophilic substitutionalcohol

+ OH-

RCH3CH2OH + Br-

(aqueous)

RCH2CH2X

alkene

hydroxide acts as a base

hydroxide acts as a nucleophile

Substitution or Elimination?

AS Chemistry

Pros and Cons

Learning ObjectivesCandidates should be able to:

interpret the different reactivities of halogenoalkanes e.g. CFCs; anaesthetics; flame retardants; plastics with particular reference to hydrolysis and to the relative strengths of the C-Hal bonds;

explain the uses of fluoroalkanes and hydrofluorooalkanes in terms of their relative chemical inertness;

recognise the concern about the effect of chlorofluoroalkanes on the ozone layer.

 

Starter activity

Chlorofluorocarbons - CFCs

. Properties:

Non-flammable

Low toxicity

Unreactive

Liquefy easily when compressed

Uses

Refrigerants

Propellants for aerosols

Solvents (including dry-cleaning)Degreasers

The ozone layer

Natural ozone layer

Replacements

•Hydrochlorofluorocarbons, HCFCs: shorter life in the atmosphere.

•Hydrofluorocarbons, HFCs: don’t contain chlorine so zero affect on ozone layer.

•Hydrocarbons: zero effect on ozone layer but flammable and lead to photochemical smog.

C. Why is BCF good at extinguishing fires?

The presence of a bromine confers flame – retarding qualities on the product.

The high temperature in fires break this compound down, producing free radicals such as Br∙. These react with other free radicals produced during combustion, quenching the flames.