Alkenes

Question 1

Define alkene

Answer 1

An unsaturated hydrocarbon with a least 1 carbon to carbon double bond

Question 2

What is the general formula of an aliphatic alkene

Answer 2

CnH2n

Question 3

Which alkanes don’t follow the general formula

Answer 3

Cyclic alkenes and alkenes with more than 1 double bond

Question 4

What are the two types of bonds called in the double bond

Answer 4

Sigma and pi

Question 5

Where’s the sigma bond in the double bond

Describe the electrons

Answer 5

X
C———C
X

Equal from each carbon atom one above and one the line

Question 6

Describe the p bond in the double bond

Answer 6

Made from two sideways overlapping p orbitals from each carbon atom each carbon atom contributes 1 electron one pi bond consists of 2 regions one above the molecule and one below. This is why the molecule is flat (planar ) around the bond

Question 7

Which bond stops rotation

Answer 7

The pi bond

Question 8

What shape is around the double bond

Answer 8

Trigonal planar

Question 9

Why is the region around the bond trigonal planar

Answer 9

There are three regions of electron density around each of the carbon atoms
The three regions repel each other as far as possible
All the atoms are in the same plane

Question 10

What is the bond angle around a double bond

Answer 10

120 degrees

Question 11

What are stereoisomers

Answer 11

They have the same structural formula but a different arrangement of atoms in space

Question 12

How does stereoisomerism arise

Answer 12

Rotation around the double bond is restricted and the groups attached to each carbon atom are therefore fixed relative to each other

Question 13

What are the conditions for E/Z isomerism

Answer 13

A c=c bond

Attached to different groups (has to be one of a group either side of the bond)

Question 14

Which one (E or Z) is for when the groups are on the same side of the double bond

Answer 14

Z (Zame side)

E Ecross

Question 15

Which group gets priority to decide if it’s a E or Z isomer

Answer 15

The highest atomic number on each side of the double bond

Question 16

Do E/Z isomers have the same or different chemical properties and why?

Answer 16

The same because they have the same functional group

Question 17

What’s the difference between e/z isomers and cis/trans isomers

Answer 17

Cis/trans has to have hydrogen

Question 18

What is needed for hydrogenation of alkenes

Answer 18

Nickel catalyst
150 degrees
H2 under pressure

Question 19

What are the conditions in halogenation

Answer 19

Rtp

Question 20

What are the conditions of halogenation with a hydrogen halide

Answer 20

Rtp

Concentrated Hx

Question 21

What is Markovnikoff’s rule

Answer 21

When a molecule of Hx reacts with a non symmetrical alkene the major product is the one in which the H-atom attaches itself to the carbon atom already carrying a larger number of hydrogens

Question 22

Which carbocation is the most stable and why
A) primary
B) secondary
C) tertiary

Answer 22

C) tertiary because each alkyl group donates and pushes electrons towards the positive charge of the carbocation. The positive charge is spread over the alkyl groups . The more alkyl groups attached to the positively charged carbon atom, the more the charge is spread out, making the ion more stable

Question 23

What is the mechanism called for alkenes

Answer 23

Electrophilic addition

Question 24

What is an electrophile

Answer 24

An electron pair acceptor

Question 25

Why does the double bond in alkenes have a high electron density

Answer 25

Because of the presence of pi electrons

Question 26

What is attracted to the high electron density

Answer 26

Electrophiles