Alkane alkene alkyne

transcript

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 –

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)

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

Alkanes may also be branched.

Hydrocarbon names are based on:

1) Type

2)No. of carbons

3) Side chain type and position

1) name will end in -ane, -ene, or -yne

2) 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

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.

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.

Example: name the following structure

CH3 CH2 C

Step 1 – Identify the correct functional

CH3 CH2 C

Step 2 - find the longest chain

CH3 CH2 C

Step 3 - add the prefix naming the longest

CH3 CH2 C

Step 4 - number the longest chain

with the lowest number closest to

the double bond

CH3 CH2 C2

CH3 CH2 C

Step 5 - add that number to the

CH3 CH2 C2

CH3 CH2 C

methyl

Step 6 - Name the side chains

CH3 CH2 C

CH3 CH2 C2

CH36ethyl

methyl

Step 7 - Place the side chains in

alphabetical order & name the

compound

CH3 CH2

CH CH3

CH2CH2

CH3 CH

CH2 CH2 CH3

CH2 CH3

CH3CH2CH CH CH CH2CH CH3

CH2CH3

CH3 CH3

CH2CH2

CH2CH3

CH2 CH CH C CH3CH3

CH3 CH2

CH CH3

CH2CH2

CH3 CH

CH2 CH2 CH3

CH2 CH3

CH3CH2CH CH CH CH2CH CH3

CH2CH3

CH3 CH3

CH3 C CH CH CH3

CH2 CH2

CH2 C C

A GOOD TIME TO INTRODUCE ISOMERS

(COMPOUNDS WITH THE SAME

MOLECULAR FORMULA BUT DIFFERENT

STRUCTURAL FORMULAE)

TRY THE FOLLOWING:

We study three particular reaction

cases:

Substitution

Addition

Elimination

Combustion

Substitution (of H,

commonly by Cl or Br)

Combustion (conversion to

CO2 & H2O)

Combustion

When alkanes are heated in a plentiful supply of air, combustion occurs

Alkanes are energetically unstable with respect to water and carbon dioxide

They 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)

Reactions with chlorine

Alkanes 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 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

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

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. H2SO4

R-CH2-CH2-OH R-CH=CH2 + H2O

ii) Dehydrohalogenation of haloalkanes

NaOH/ethanolR-CH2-CH2-X

refluxR-CH=CH2 + HX

NaOH can be replaced by KOH

CH3CH2-CH-CH3

CH3CH=CH-CH3 + H2O

CH3CH2-CH=CH2 + H2O

2-butanol2-butenemajor product

1-butene

Dehydration of alcohols

Dehydrohalogenation of haloalkanes

CH3CH-CH-CH2

KOH CH3CH=CH-CH3 CH3CH2CH=CH2alcohol

reflux

2-bromobutane2-butene

(major product)1-butene

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 C

Pt or Pd

25-90oC

- 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

- 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

inert solvent

X X = halogen such as Br2 or Cl2

Inert solvent = CCl4 or CH2Cl2

EXAMPLES:

H H Br Br

CH3CH=CH2 Cl2CCl4

inert solvent (CCl4)

ethene1,2-dibromoethane

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

solution becomes colourless.

cyclohexene1,2-dibromocyclohexane

propene 1,2-dichloropropane

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.

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.

- the alkene is absorbed slowly when it

passed through concentrated sulfuric acid in the cold (0-15oC

• 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

ethene ethanol

C C H2O C C

alkene alcohol

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

Functional group = halogen

› Ex. Fluorine = fluoro

Number by which carbon attached to,

put in alphabetical order

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

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.

When an aqueous solution of NaOH or KOH is added

to haloalkane an alcohol is produced.

propan-2-ol

Select the longest chain which contains

the OH group and number so that the

OH group has the smallest number. See

the examples below

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

CH3 CH2 CH2 CH CH3

CH3 CH2 CH2 C OH

CH3 CH2 CH2 CH2 OH

Alcohols contain an –OH group covalently

bonded to a carbon atom.

We need know:

the esterification reaction

Substitution and

elimination

1. By hydration of alkanes

The acid is absorbed in conc sulphuric

acid and then the acid is diluted.

2. Hydrolysis of halogenoalkanes

The halogen of the halogenoalkane is

replaced by an OH group Refer to

Halogenoalkanes

CH3CHCH3

OHH2SO4

CH2 CHCH3

PtCH3CH2CH3

alcohol alkene alkane

Acid + Alcohol yields Ester + Water

Sulfuric acid is a catalyst.

Each step is reversible.

CH3 C OH

+ CH2CH2CHCH3

OCH2CH2CHCH3

Acid + Alcohol yields Ester + Water

Sulfuric acid is a catalyst.

Each step is reversible.

Chapter 11 71

CH3 C OH

+ CH2CH2CHCH3

OCH2CH2CHCH3

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

Alkane alkene alkyne

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