Alkanes, alkenes &alkynes

ALKANES, ALKENES, ALKYNES

NOMENCLATURE, STRUCTURES AND ISOMERISM

Alkanes = CnH2n+2

Alkenes = CnH2n

Alkynes = CnH2n-2

ALKANES, ALKENES, ALKYNES AND CYCLOALKANES ARE HYDROCARBONS (COMPOUNDS CONTAINING ONLY CARBON AND HYDROGEN).

EACH OF THESE FORM A HOMOLOGOUS SERIES (A GROUP OF ORGANIC COMPOUNDS HAVING A COMMON GENERAL FORMULA/ OR IN WHICH EACH MEMBER FIFFERS FROM THE NEXT BY A –CH2)

THE HYDROCARBONS MAY BE SATURATED (CONTAINS ONLY SINGLE BONDS BETWEEN CARBON-CARBON ATOMS/ CARBON ATOMS BONDED TO THE MAXIMUM NUMBER OF HYDROGENS)

OR UNSATURATED (CONTAINS AT LEAST A DOUBLE BOND BETWEEN C-C ATOMS)

ALKANES: NOMENCLATURE

Also called paraffins.A group of saturated hydrocarbons with the

general formula Cn H2n+2 .They form a homologous series.Straight chain alkanes have their carbon

atoms bonded together to give a single chain

Alkanes may also be branched.

NAMING (GENERAL)

Hydrocarbon names are based on: 1) type, 2) # of carbons, 3) side chain type and position

1) name will end in -ane, -ene, or -yne2) the number of carbons is given by a “prefix”

1 meth- 2 eth- 3 prop- 4 but- 5 pent- 6 hex- 7 hept- 8 oct- 9 non- 10 dec-

Actually, all end in a, but a is dropped when next to a vowel. E.g. a 6 C alkene is hexene

Determine the longest continuous chain (not always straight) in the molecule. The base name of the hydrocarbon is the name of the longest chain.

IUPAC system

IUPAC SYSTEM

Name any chain branching off the longest chain as an alkyl group (e.g., methyl, ethyl etc)

The complete name of a branch requires a number that locates the branch on the longest chain.

Therefore number the chain in whichever direction gives the smaller number for all branches.

6. When two or more branches are identical, use prefixes (di-, tri-, etc.) (e.g. 2,4-dimethylhexane). Numbers are separated with commas. Prefixes are ignored when determining alphabetical order. (e.g. 2,3,5-trimethyl-4-propylheptane)

7. When identical groups are on the same carbon, repeat the number of this carbon in the name. (e.g. 2,2-dimethylhexane)

Where there are two or more different alkyl branches, the name of each branch, with its position number precedes the name. the branch names are placed in alphabetical order.

Alkenes and alkynes

Both groups are unsaturated hydrocarbons. Each group is a homologous series.The main chain is defined as the chain

containing the greatest number of double/tripple bonds

We number the position of the double/tripple bond so that it has the lowest numbers.

ALKENES

alkynes

Naming side chainsExample: name the following structure

CH3 CH2 C

CH2

CH2 C

CH2

CH3

CH3

CH3

Step 1 – Identify the correct functional group

Naming side chains

CH3 CH2 C

CH2

CH2 C

CH2

CH3

CH3

CH3

Step 2 - find the longest chain

Naming side chains

CH3 CH2 C

CH2

CH2 C

CH2

CH3

CH3

CH3

Step 3 - add the prefix naming the longest chain

Naming side chains

CH3 CH2 C

CH2

CH2 C

CH2

CH3

CH3

CH3

Step 4 - number the longest chain with the lowest number closest to the double bond

CH3 CH2 C2

CH21

CH23

C4

CH25

CH3

CH3

CH36

CH3 CH2 C

CH2

CH2 C

CH2

CH3

CH3

CH3

Naming side chains

Step 5 - add that number to the name

CH3 CH2 C2

CH21

CH23

C4

CH25

CH3

CH3

CH36

CH3 CH2 C

CH2

CH2 C

CH2

CH3

CH3

CH3

Naming side chains

ethyl

methyl

methylStep 6 - Name the side chains

CH3 CH2 C

CH2

CH2 C

CH2

CH3

CH3

CH3

CH3 CH2 C2

CH21

CH23

C4

CH25

CH3

CH3

CH36

Naming side chains

ethyl

methyl

methylStep 7 - Place the side chains in alphabetical order & name the compound

name the following

CH3 CH2CH CH3

CH2CH2

CH3

CH3 CH

CH

CH3

CH

CH3

CH2 CH2 CH3

CH2 CH3

CH3CH2CH CH CH CH2CH CH3

CH3

CH2CH3

CH3 CH3

CH3

CH2CH2

CH2CH2

CH2CH2

CH3

CH3

CHCH2

CH2CH

CH2CH2

CH3

CH3

CH3

CH2

CHCH2

CHCH2

CH2

CH3

CH2 CH3

CH2 CH CH C CH3CH3

CH3

CH3

1

2

3

4

CH3 CH2CH CH3

CH2CH2

CH3

CH3 CH

CH

CH3

CH

CH3

CH2 CH2 CH3

CH2 CH3

CH3CH2CH CH CH CH2CH CH3

CH3

CH2CH3

CH3 CH3

9 10

11

CH3 C CH CH CH3

CH2 CH2

CH3

CH3

CH CH

CH2

CH

CH3

CH3

CH3

CH2 C CCH2

CH3

CH3

ISOMERS

A GOOD TIME TO INTRODUCE ISOMERS (COMPOUNDS WITH THE SAME MOLECULAR FORMULA BUT DIFFERENT STRUCTURAL FORMULAE)

TRY THE FOLLOWING:

Reactions of alkanes & alkenes

We study three particular reaction cases:

SubstitutionAdditionEliminationCombustion

Reactions of alkanes

Substitution (of H, commonly by Cl or Br)Combustion (conversion to CO2 & H2O)

CombustionWhen alkanes are heated in a plentiful

supply of air, combustion occursAlkanes are energetically unstable with

respect to water and carbon dioxideThey only burn when they are in the

gaseous state Explain what happens when a candle burns!

2 C4H10(g) + 13 O2(g) 8 CO2(g) + 10 H2O(g)

2 C8H18(l) + 25 O2(g) 16 CO2(g) + 18 H2O(g)

SUBSTITUTION

Reactions with chlorineAlkanes only react with chlorine when a

mixture of the two is exposed to sunlight or ultraviolet light

The light provides the energy required to break the very strong bonds

This is an example of a substitution reaction

In the presence of light, or at high temperatures, alkanes react with halogens to form alkyl halides. Reaction with chlorine gives an alkyl chloride.

CH4(g) + Cl2(g) CH3Cl(g) + HCl(g)

Cracking

Cracking happens when alkanes are heated in the absence of air

The products of the cracking of long-chain hydrocarbons are shorter chain molecules

Ethane is cracked industrially to produce ethene

PHYSICAL PROPERTIES

Alkanes are non polar so they are insoluble in water but soluble in each other.

Low molecular alkanes are gases.Boiling points increase with increasing

chain length (molecular weight) for the first few members

Boiling points decrease with increasing number of branches.(Explain this in terms of Van der Waals’ forces and surface area.

Melting and boiling points increase with increased molecular weight (Methane bp.

-164°C, decane bp. 174°C)While boiling point decrease with chain

branching (decrease in surface area), melting

points increase· Alkanes are less dense than water and

swim on top of water

alkenes: preparation and reactions

Alkenes: Preparation and reactions

Two ways of making alkenes:1. Heat a concentrated solution of potasium

/sodium hydroxide in alcohol (alcoholic KOH) with a haloalkane (halogenoalkane)

This is dehydrohalogenation (removal of hydrogen and halogen)

2. Heat concentrated sulphuric acid with the alcohol- dehydration. THE ACID IS A DEHYDRATING AGENT

i) Dehydration of alcohols

conc. H2SO4R-CH2-CH2-OH R-CH=CH2 + H2O

ii) Dehydrohalogenation of haloalkanesNaOH/ethanolR-CH2-CH2-X reflux R-CH=CH2 + HX

NaOH can be replaced by KOH

LEARNERS MUST KNOW MAJOR PRODUCTS IN ALL CASES AND REACTION CONDITIONS

CH3CH2-CH-CH3OH

H+

H+

CH3CH=CH-CH3 + H2O

CH3CH2-CH=CH2 + H2O

2-butanol2-butenemajor product

1-butene

Dehydration of alcohols

Dehydrohalogenation of haloalkanes

CH3CH-CH-CH2

BrH HKOH CH3CH=CH-CH3 CH3CH2CH=CH2

alcoholreflux

2-bromobutane2-butene(major product) 1-butene

REACTIONS OF ALKENES (VIP)

Catalytic hydrogenation:- hydrogenation: addition of hydrogen

to a double bond and triple bond to yield saturated product.

- alkenes will combine with hydrogen in the present to catalyst to form alkanes.

C C H H C CH H

Pt or Pd25-90oC

- Plantinum (Pt) and palladium (Pd) – Catalysts - Pt and Pd: temperature 25-90oC- Nickel can also used as a catalyst, but a higher

temperature of 140oC – 200oC is needed.

Addition of halogens:

i) In inert solvent:- alkenes react with halogens at room temperature and in dark.- the halogens is usually dissolved in an inert solvent such as dichloromethane (CH2Cl2) and tetrachloromethane (CCl4).- Iodine will not react with alkenes because it is less reactive than chlorine and bromine.- Fluorine is very reactive. The reaction will produce explosion. C C X X C C

X X

inert solvent

X X = halogen such as Br2 or Cl2Inert solvent = CCl4 or CH2Cl2

EXAMPLES:

C CHH

H H Br Br

Br2

Br

Br

CCl4

CH3CH=CH2 Cl2CCl4 CH3CH

ClCH2

Cl

C CBr

H H

BrH H

inert solvent (CCl4)

ethene1,2-dibromoethane

* the red-brown colour of the bromine solution will fade and the solution becomes colourless.

cyclohexene 1,2-dibromocyclohexane

propene 1,2-dichloropropane

Hydrohalogenation

MARKOVNIKOV’S RULE ( A statement of the rule is not needed)

There are 2 possible products when hydrogen halides react with an unsymmetrical alkene.

It is because hydrogen halide molecule can add to the C=C bond in two different ways.

C C

H

HCH3

H

H-I

C C

H

HCH3

H

H-I

C C

H

HCH3

H

H I

C C

H

HCH3

H

I H

1-iodopropane

2-iodopropane(major product)

Markovnikov’s rules (Not for examination)

- the addition of HX to an unsymmetrical alkene, the hydrogen atom attaches itself to the carbon atom (of the double bond) with the larger number of hydrogen atoms.

Addition reaction with concentrated sulfuric acid: hydration of alkenes

- the alkene is absorbed slowly when it passed through concentrated sulfuric acid in the cold (0-15oC

Addition reaction with acidified water (H3O+): hydration of alkenes

• Hydration: The addition of H atoms and –OH groups from water molecules to a multiple bond.

• Reverse of the dehydration reaction.• Direct hydration of ethene: - passing a mixture of ethene and

steam over phosphoric (v) acid (H3PO4) absorbed on silica pellets at 300oC and a pressure of 60 atmospheres.

- H3PO4 is a catalyst.

CH2=CH2 H2OH3PO4 CH3CH2OH(g) (g)

300 oC, 60 atm(g)

ethene ethanol

C C H2O C CH OH

alkene alcohol

Oxidation (Combustion of alkenes)

The alkenes are highly flammable and burn readily in air, forming carbon dioxide and water.

For example, ethene burns as follows :

C2H4 + 3O2 → 2CO2 + 2H2O

HALOGENOALKANESHalogenoalkanes are compounds in which one

or more hydrogen atoms in an alkane have been replaced by halogen atoms (fluorine,

chlorine, bromine or iodine).

Functional group = halogen◦Ex. Fluorine = fluoro

Number by which carbon attached to, put in alphabetical order

Ex.

Bromoethane

Halogenoalkanes fall into different classes depending on how the halogen atom is positioned on the chain of carbon atoms. There are some chemical differences between the various types.

• Primary

• Secondary

• Tertiary

◦Primary (1°) – carbon carrying halogen is attached to only one carbon alkyl group

◦Secondary (2°)– carbon carrying halogen is attached to two other alkyl groups

◦Tertiary (3°) – carbon carrying halogen is attached to three alkyl groups

Reactions of the halogenoalkanes

Substitution:In a substitution reaction, one atom or group of atoms, takes the place of another in a molecule.

Elimination: Halogenoalkanes also undergo elimination reactions in the presence of sodium or potassium hydroxide which is

dissolved in ethanol.

Example of substitution

When an aqueous solution of NaOH or KOH is added to haloalkane an alcohol is produced.

propan-2-ol

Example of eliminationwhat conditions are needed?

ALCOHOLS (CnH2n+1OH)Preparation and properties

nomenclature

Select the longest chain which contains the OH group and number so that the OH group has the smallest number. See the examples below

Classification

In a primary (1°) alcohol, the carbon which carries the -OH group is only attached to one alkyl group.

In a secondary (2°) alcohol, the carbon with the -OH group attached is joined directly to two alkyl groups, which may be the same or different.

In a tertiary (3°) alcohol, the carbon atom holding the -OH group is attached directly to three alkyl groups, which may be any combination of same or different.

See the examples below

Alcohols are classified as primary, secondary or Tertiary

CH3 CH2 CH2 CH CH3

OH

CH3 CH2 CH2 C OH

CH3

CH3

CH3 CH2 CH2 CH2 OH

Reactions of alcohols

Alcohols contain an –OH group covalently bonded to a carbon atom.

We need know: the esterification reactionSubstitution and elimination

Preparation and reactions

1. By hydration of alkanesThe acid is absorbed in conc sulphuric

acid and then the acid is diluted.

2. Hydrolysis of halogenoalkanesThe halogen of the halogenoalkane is

replaced by an OH group Refer to Halogenoalkanes

Classic example for learners to write

CH3CHCH3

OHH2SO4

CH2 CHCH3H2

PtCH3CH2CH3

alcohol alkene alkane

esterification

Acid + Alcohol yields Ester + WaterSulfuric acid is a catalyst.Each step is reversible.

CH3 C OH

O

+ CH2CH2CHCH3

CH3

OHH+

CH3C

O

OCH2CH2CHCH3

CH3

+ HOH

=>

Chapter 11 72

Acid + Alcohol yields Ester + WaterSulfuric acid is a catalyst.Each step is reversible.

CH3 C OH

O

+ CH2CH2CHCH3

CH3

OHH+

CH3C

O

OCH2CH2CHCH3

CH3

+ HOH

=>

Aldehydes andKetones (Know the functional groups)

Nomenclature of Aldehydes: Select the longest carbon chain containing the carbonyl carbon. • The -e ending of the parent alkane name is

replaced by the suffix -al. • The carbonyl carbon is always numbered “1.” (It is not necessary to include the number in the name.) • Name the substituents attached to the chain in the usual way

Nomenclature of Ketones

No reactions. Just naming

SOME FUNCTIONAL GROUPS TO KNOW