Alkenes

Question 1

Describe the structure of alkenes?

Answer 1

• unsaturated hydrocarbons
• their general formula is CnH2n

Question 2

What is a sigma bond?

Answer 2

The overlap of two orbitals consisting of a pair of electrons

Question 3

What is a pi bond?

Answer 3

• the overlap of two p orbitals

Question 4

Explain the bonds in the double bond of alkenes

Answer 4

• three out of four electrons are used in three sigma bonds to other atoms, this leaves one electron in a p oribtal
• this forms a pi bond as the p orbitals from each carbons overlap

Question 5

What does the P orbital make alkenes do

Answer 5

The pi bond prevents rotation around the double bond

Question 6

What is the shape around a double bond

Answer 6

There are three areas of electron density, these repel causing a bond angle of 120 degrees

Question 7

Define stereoisomers

Answer 7

Compounds with the same structural formula but with different arrangements of atoms in space

Question 8

Why do stereoisomers exist?

Answer 8

You cant rotate around the carbon double bond

Question 9

What conditions must an alkene meet to have stereoisomers?

Answer 9

• a double carbon to carbon bond
• different groups attached to each carbon atom of the double bond

Question 10

What is an E isomer?

Answer 10

• if the atoms with the highest atomic number are on opposite sides

Question 11

What is a Z isomer?

Answer 11

Used when atoms with the highest atomic number are attached on the same side of the bond

Question 12

Who made these priority rules?

Answer 12

Cahn - Ingold - prelog rules

Question 13

What are cis - trans isomers?

Answer 13

Special cases of E/Z isomers where the two carbon atoms within a double bond have the same group

Question 14

What are cis/trans isomers equivalent to ?

Answer 14

E = trans
Z = Cis

Question 15

What are the three reactions that alkenes undergo?

Answer 15

• Combustion
• electrophilic addition
• addition polymerisation

Question 16

What are the different electrophilic addition reactions alkenes undergo?

Answer 16

-hydrogen
- halogens
- hydrogen halides
- steam

Question 17

What are the conditions and products of the hydrogenation of alkenes?

Answer 17

• forms an alkane
• nickel catalyst

Question 18

What are the products of an addition reaction of halogens with alkenes?

Answer 18

• a dihaloalkane

Question 19

What is the test for alkenes

Answer 19

• add bromine water to an unsaturated compound
• the bromine reacts with the double bond, forming a haloalkane and causing a colour change from orange to colourless

Question 20

What are the products of an addition reaction of hydrogen halides and alkenes?

Answer 20

• a haloalkane

Question 21

What are the products and conditions of steam and alkenes

Answer 21

• an alcohol
• requires an acid catalyst and high temps and pressures

Question 22

What is a electrophile?

Answer 22

an electron pair acceptor

Question 23

Draw the mechanism of electrophilic addition for propene and Br2

Question 24

What is Markownikoffs rule?

Answer 24

When a hydrogen halide reacts with an unsymmetrical alkene, the hydrogen attaches itself to the carbocation with the smallest amount of hydrogens and largest amount of carbons

Question 25

Why does the halogen attach to the carbocation with the most carbons

Answer 25

- due to carbocation stability - the more alkyl groups a carbocation is attached to, the more stable it is

Question 26

describe a primary, secondary and tertiary carbocation?

Answer 26

Primary - 1 other carbon Secondary - 2 Tertiary - 3

Question 27

How can we determine what will be major product?

Answer 27

the carbocation with the most carbons attached will be the major product