AS Chemistry

Bonding in methane, ethane and ethene



and



bonds

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 hex-3-ene 2,3-dimethylpent-2-ene CH 3 CH=CHCH 2 CH 3 CH 3 CH 2 CH=CHCH 3 cyclopenta-1,3-diene 3-ethylhept-1-ene CH 2 =CHCH 2 CH(CH 2 CH 3 )CH 2 CH 2 CH 3

Hybridisation of orbitals

The electronic configuration of a carbon atom is

1s 2 2s 2 2p 2 2 2p 2s 1 1s

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

1s 2 2s 1 2p 3 2 2p 2s 1 1s

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

Hybridisation of orbitals in alkanes

The 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

hybridisation.

sp 3 2s 2 2p 2 2s 1 2p 3 4 x sp 3

Hybridisation of orbitals in alkanes

In

ALKANES

orbitals repel each other into a , the four sp 3 tetrahedral arrangement.

sp 3 orbitals

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.

2s 2 2p 2 2s 1 2p 3 3 x sp 2 2p

What about ethene?

sp 2 hybrids



- bonds

AS Chemistry

Geometric Isomerism

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

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 ISOMERISM STRUCTURAL ISOMERISM STEREOISOMERISM GEOMETRIC ISOMERISM OPTICAL ISOMERISM

Geometric Isomerism?

GEOMETRIC ISOMERISM RESTRICTED ROTATION OF C=C BONDS Single covalent bonds can easily rotate same. . What appears to be a different structure in an alkane is not. Due to the way structures are written out, they are the 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

Geometric Isomerism?

X



GEOMETRIC ISOMERISM How to tell if it exists Two different atoms/groups attached Two similar atoms/groups attached Two similar atoms/groups attached Two different atoms/groups attached Two similar atoms/groups attached Two different atoms/groups attached

  

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



GEOMETRICAL ISOMERISM

GEOMETRIC ISOMERISM Isomerism in butene There are 3 structural isomers of C 4 H 8 that are alkenes * . Of these ONLY ONE exhibits geometrical isomerism.

BUT-1-ENE cis BUT-2-ENE trans BUT-2-ENE 2-METHYLPROPENE * YOU CAN GET ALKANES WITH FORMULA C 4 H 8 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 isomer

cis

melting point ( ° C)

-80

boiling ( ° C) 60 point

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.

AS Chemistry

Optical Isomerism

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

Chiral centre Chiral molecule 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.

Optical Isomerism What 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 Isomerism The polarimeter A B C D E F A B C D E F Light source produces light vibrating in all directions Polarising filter only allows through light vibrating in one direction Plane polarised light passes through sample If substance is optically active it rotates the plane polarised light Analysing filter is turned so that light reaches a maximum Direction of rotation is measured coming towards the observer If the light appears to have turned to the right DEXTROROTATORY turned to the left LAEVOROTATORY

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 CHAIN ISOMERISM

STRUCTURAL ISOMERISM

Same molecular formula but different structural formulae POSITION ISOMERISM FUNCTIONAL GROUP ISOMERISM

STEREOISOMERISM

Same molecular formula but atoms occupy different positions in space.

GEOMETRICAL ISOMERISM Occurs due to the restricted rotation of C=C double bonds... two forms - CIS and TRANS OPTICAL ISOMERISM Occurs when molecules have a chiral centre. Get two non superimposable mirror images.

AS Chemistry

Electrophilic Addition to Alkenes

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

Electrophilic addition

CH 2 =CH 2 + Br 2  CH 2 BrCH 2 Br

bromine with ethene

CH 2 =CH 2 + Br 2 CH 2 Br CH 2 Br 1,2-dibromoethane

hydrogen bromide with ethene

CH 2 =CH 2 + H Br C H 3 CH 2 Br bromoethane

Electrophilic addition mechanism

H H C H C H Br  + Br  H H H C

carbocation

H Br Br Br Br 1,2-dibromoethane H H C H C Br Br H

Electrophilic addition mechanism

H H C H  + Br  H C H H H H C Br H carbocation H H H C H C H Br H bromoethane

Electron flow during electrophilic addition

EQUATION TEMPERATURE ( O C) PRESSURE CATALYST PHASE NOTES

hydrogen CH 2 =CH 2 + H 2 → CH 3 CH 3 steam CH 2 =CH 2 + H 2 O → CH 3 CH 2 OH hydrogen halides (e.g. HBr) CH 2 =CH 2 + HBr → CH 3 CH 2 Br halogens CH 2 =CH 2 +Br 2 → CH 2 BrCH 2 Br ~150 330 6MPa Finely divided nickel on support material Phosphoric (V) acid silica.

(H 3 PO 4 ) adsorbed onto the surface of Gas Gas Never carried out industrially.

Analogous reaction used to produce some margarines from oils (see later).

Major industrial process for the manufacture of ethanol.

Aqueous solution Reactivity increases from HF to HI.

Room temperature Room temperature Liquid bromine or and solvent.

solution (both aqueous non-polar Chlorine products.

powerful and iodine produce similar addition Fluorine is too an oxidizing agent.

Addition reactions of alkenes

Addition to unsymmetrical alkenes Electrophilic addition to propene

2-bromopropane 1-bromopropane

Addition to unsymmetrical alkenes In the electrophilic addition to alkenes the major product is formed via the more stable carbocation (carbonium ion) least stable methyl < most stable primary (1°) < secondary (2°) < tertiary (3°)

Addition to unsymmetrical alkenes SECONDARY CARBOCATION PATH A MAJOR PRODUCT PRIMARY CARBOCATION PATH B MINOR PRODUCT

AS Chemistry

Polymerisation

Learning Objectives Candidates should be able to:

 describe the chemistry of alkenes including polymerisation.

 describe PVC.

the characteristics of addition polymerisation as exemplified by poly(ethene) and  Recognize the difficulty of the disposal of poly(alkene)s, i.e. non-biodegradability and harmful combustion products.

Starter activity

Poly(ethene)

Conditions

Temperature: Pressure: Initiator:

about 200 ° C about 2000 atmospheres often a small amount of oxygen as an impurity

Free radical addition

Initiation Propagation Propagation Termination

LDPE or HDPE

LDPE or HDPE

Sandwich bags, cling wrap, car covers, squeeze bottles, liners for tanks and ponds, moisture barriers in construction Freezer bags, water pipes, wire and cable insulation, extrusion coating

Polymerisation of alkenes ETHENE POLY(ETHENE) CHLOROETHENE POLY(CHLOROETHENE) POLYVINYLCHLORIDE PVC PROPENE TETRAFLUOROETHENE POLY(PROPENE) POLY(TETRAFLUOROETHENE) PTFE “Teflon”

Disposal of polymers Method

Landfill Incineration Recycling Feedstock recycling

Comments

Emissions to the atmosphere and water; vermin; unsightly. Can make use of old quarries.

Saves on landfill sites and energy.

May also release toxic and greenhouse gases.

high cost processing.

produces of collection and re Use the waste for the production of useful organic back into polymers.

compounds.

New technology can convert waste into hydrocarbons which can then be turned

AS Chemistry

Oxidation of alkenes

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.

CH 2 =CH 2 + H 2 O + [O]  HOCH 2 CH 2 OH ethane – 1,2-diol

Oxidation of alkenes

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.

Fragment Product

=CH 2 CO 2 R-CH= Aldehyde → carboxylic acid R 2 C= Ketone

Oxidation of alkenes

1. CH 2 =CH 2 2. CH 3 CH=CH 2 3. (CH 3 ) 2 C=CH 2 2 products – both contain ketone 1 product only 2 products – one contains 2 ketone groups and one contains 2 acid groups.

AS Chemistry

Halogenoalkanes

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

Naming Halogenoalkanes

a. CHCl 3 b. CH 3 CHClCH 3 c. CF 3 CCl 3

trichloromethane 2-chloropropane 1,1,1-trichloro-2,2,2-trifluoroethane

F F Cl F Cl Cl

Physical Properties

a. 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 S

N

2 reaction.

 S stands for substitution,  N for nucleophilic, and  2 because the initial stage of the reaction involves two species.

Nucleophilic substitution - mechanism

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

–

ANIMATION SHOWING THE S N 2 MECHANISM

Rate of reaction Halogen

Electronegativity Bond strength (C-X) kJ mol -1

F

4.0

484

Cl

3.0

338

Br

2.8

276

I

2.5

238 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.

Measuring the rate of reaction Experiment

Water is a poor nucleophile but it can slowly displace halide ions

C 2 H 5 Br (l) + H 2 O (l)



C 2 H 5 OH (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.

hydroxide ion with bromoethane

CH 3 CH 2 Br + OH ( aqueous ) warm CH 3 CH 2 OH ethanol + Br -

Water with bromoethane

CH 3 CH 2 Br + H 2 O ( aqueous ) warm CH 3 CH 2 OH ethanol + HBr This is a slower reaction – water is not such a good nucleophile.

Nucleophilic substitution mechanism

H CH 3 

+

C H OH 

-

CH 3 H C H ethanol OH Br -

Nucleophilic substitution mechanism

CH  3 + H C H H 2 O  CH 3 H C H + OH H ethanol CH 3 H C H OH HBr Br -

Nucleophilic substitution cyanide ion with bromoethane

CH 3 CH 2 Br + CN (ethanol) reflux CH 3 CH 2 CN + Br propanenitrile -

ammonia with bromoethane

CH 3 CH 2 Br + NH 3 (ethanol) Heat / pressure CH 3 CH 2 Br + NH 3 (ethanol) Heat / pressure CH 3 CH 2 NH 2 aminoethane + HBr CH 3 CH 2 NH 2 + NH 4 + Br -

Nucleophilic substitution mechanism

CH  3

+

H C H CN 

-

CH 3 H C CN H propanenitrile Br -

Nucleophilic substitution mechanism

CH  3 + H C H NH 3  CH 3 H C H + NH 2 H Br NH 3 CH 3 aminoethane H C H NH 2 H NH 3 + Br -

Past paper question

Cl 2 U.V. /sunlight Ethanolic KCN reflux Br 2 U.V. /sunlight

AS Chemistry

Substitution vs. Elimination

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 S N 1 and S N 2 mechanisms) in halogenoalkanes.

Starter activity

Type of halogenoalkane

primary

Position of halogeno- group

at end of chain: bromoethane

Example

secondary in middle of chain: 2-bromopropane tertiary attached to a carbon atom which carries no H atoms: 2-bromo-2-methylpropane

S N 1 – tertiary halogenoalkanes

Nucleophilic attack at the back of the molecule is hindered by bulky CH 3 groups. Tertiary carbocation is stabilised by electron donating effect of CH 3 groups.

S N 1 or S N 2 ? Halogenoalkane

Primary Secondary Tertiary

Mechanism

S N 2 S N 1 and S N 2 S N 1

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.

CH 3 CH 2 Br + NaOH  CH 2 =CH 2 + NaBr + H 2 O

Elimination of HBr from 2-bromopropane

CH 3 CHBrCH 3 + OH ( in

ethanol

) CH 3 CH = CH 2 + H 2 O + Br CH 3 H C Br H C H H H H OH acting as a

base

CH 3 C Br C H propene H OH

nucleophilic substitution alcohol RCH 3 CH 2 OH + Br + OH ( aqueous ) RCH 2 CH 2 X + OH ( ethanol ) elimination hydroxide acts as a

nucleophile

hydroxide acts as a

base

RCH = CH 2 alkene + H 2 O + X 92

AS Chemistry

Pros and Cons

Learning Objectives

Candidates should be able to:  interpret bonds; 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  explain the chemical inertness; uses of fluoroalkanes and hydrofluorooalkanes in terms of their relative  recognise the concern about the effect of chlorofluoroalkanes on the ozone layer.

Starter activity

. Properties:  Non-flammable  Low toxicity  Unreactive  Liquefy easily when compressed

Refrigerants Propellants for aerosols Solvents (including dry-cleaning) Degreasers

 

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.