Alkenes

CSEC Chemistry

Functional group of alkenes

C=C double bond

General formula for alkene homologous series.

CnH2n

All alkenes are ___________ compounds.

unsaturated

How many carbon atoms must be present for an alkene to have structural isomers?

4 or more carbons are required. Structural isomers results from 1) change in the position of the functional group (double bond) 2) branching of the molecule.

When numbering the carbons of an alkene, it should be numbered such that the double bond gets the ________ number possible.

lowest

Name CH3CH2CHCH=CH2

pent-1-ene or 1-pentene

Name CH3CH2C(CH3)CHC=CH2

3-methylpent-1-ene

Name CH3CH2=C(CH3)CH2

2-methylbut-2-ene

Alkene + oxygen -->

carbon dioxide (g) + water (g)

Why do alkenes burn with a smoky flame?

They have a higher ration of carbon to hydrogen atoms. When they burn, they release unreacted carbon as soot and sometimes carbon monoxide in addition to carbon dioxide and steam.

What happens in an addition reaction?

If hydrogen is added to an alkene in an addition reaction, the process is called

hydrogenation

What conditions are required for hydrogenation of an alkene?

Ni catalyst, 5 atm , 150 degrees C

C2H4 (g) + H2 (g) --> (Ni catalyst, 5 atm, 150 degrees C above arrow)

C2H6 (g)

How many different products are always formed from addition reactions?

One

If a halogen is added to an alkene in an addition reaction, the process is called

halogenation

The product of halogenation reactions is called a

haloalkane

What conditions are required for halogenation of an alkene?

C3H6 (g) + Br2 (g) -->

C3H6Br2 (l)

Alkene + hydrogen halide -->

haloalkane

C2H4 (g) + HCl (g) -->

C22H5Cl (l)

What is a haloalkane/alkyl halide?

An alkane with one or more hydrogen replaced with a halogen atom.

If water is added to an alkene in an addition reaction, the process is called

hydration

What conditions are required for hydration of an alkene to occur?

H3PO4 in silica; 60-70 atm ; 300 degrees C (This is the process used in the industrial preparation of ethanol from ethene)

Alkene + oxidising agent [O] -->

dialcohol (diol)

Ethene + KMnO4 -->

ethane-1,2-diol

C2H4 (g) + H20 (l) + [O] -->

C2H4(OH)2(l)

Positive test for ALKANES using KMnO4 will give this result

Solution remains purple since alkanes fo not react with oxidising agents.

Positive test for ALKENES using KMnO4 will give this result

Solution changes colour rapidly from purple to colourless since alkenes rapidly reduce the purple MnO4 superscipt - ions to colourless Mn superscript 2+ ions

Positive test for ALKANES using bromine solution will give this result

Solution remains red-brown since alkanes do not react with bromine under standard lab conditions.

Positive test for ALKENES using bromine solution will give this result

Solution changes rapidly from red-brown to colourless since alkenes undergo rapid addition reaction with bromine solution, under any conditions, including darkness.

What are alkenes extremely useful as starting materials in manufacturing other chemicals?

They undergo a wide range of reactions (e.g. addition reactions and polymerization) due to the presence of carbon-carbon double bonds.