Alkenes

Question 1

What is a sigma bond?

Answer 1

A C-C single bond formed by the end-on overlap of p orbitals in a straight line between the bonding atoms.

Question 2

What is a pi bond?

Answer 2

A second C-C bond formed by the sideways overlap of two p orbitals above and below the plane of the existing sigma bond to create a second shared electron pair.

Question 3

Describe a C=C double bond

Answer 3

A non-rotational bond formed when a pi bond forms above and below the plane of a sigma bond to give two shared electron pairs.

Question 4

Give the relative strengths of carbon bonds.

Answer 4

pi bond < sigma bond < double bond

Question 5

Describe shape of a carbon atom engaged in a single C-C bond

Answer 5

Tetrahedral with a 109.5 degree bond angle, as there are no lone electron pairs.

Question 6

Describe shape of a carbon atom engaged in a double C=C bond

Answer 6

Trigonal planar with a 120 degree bond angle, as the carbon atom is attached to 3 equally repulsive species.

Question 7

Describe shape of a carbon atom engaged in a triple CC bond

Answer 7

Linear, with a 180 degree bond angle.

Question 8

What is a stereo isomer?

Answer 8

A molecule with the same molecular and structural formula, but a different arrangement of atoms in space.

Question 9

What are the conditions for E/Z isomerism?

Answer 9

Each carbon in part of a non-rotational double bond must be attached to two different species.

Question 10

What is an E isomer?

Answer 10

A stereo-isomer in which the major groups on the carbon atoms are on opposite sides of the molecule.

Question 11

What is a Z isomer?

Answer 11

A stereo-isomer in which the major groups on the carbon atom are on the same side of the molecule.

Question 12

How do you find the major group on a Carbon atom?

Answer 12

Count along the main chain of each group until you find an atom which differs. The atom with the greatest Mr is attached to the major group.

Question 13

When does cis/ trans isomerism occur?

Answer 13

When two carbons in a non rotational double bond share one carbon group but differ on the other.

Question 14

What is a cis isomer?

Answer 14

A molecule in which the common groups on each carbon are on the same side of the molecule

Question 15

What is a trans isomer?

Answer 15

A molecule in which the common groups on each carbon are on opposite sides.

Question 16

What can be observed when an alkene is is mixed with bromine water?

Answer 16

The decolourisation of bromine water from orange to colourless.

Question 17

Define an electrophile.

Answer 17

An electron deficient species which can accept a pair of electrons to form a new covalent bond.

Question 18

What does a curly arrow represent?

Answer 18

The movement of a pair of electrons

Question 19

What is heterolytic fission?

Answer 19

When a bond breaks and one bonding atom takes both the bonding electrons.

Question 20

What is electrophilic addition?

Answer 20

The mechanism by which an electrophile breaks apart a double bond and is added to an organic molecule.

Question 21

Name the process of addition to alkenes

Answer 21

Electrophilic addition

Question 22

How can a hydrogen halide act as an electrophile?

Answer 22

H-X has a polar bond, so the H atom has a delta + charge, allowing it to attract electrons.

Question 23

Describe the first step of electrophilic addition between an alkene and a hydrogen halide.

Answer 23

The delta + charge on the hydrogen atom in H-X attracts the electron pair in the C=C pi bond. HX splits heterolytically, and hydrogen accepts the electron pair in the C=C bond to give a carbocation and a negatively charged halogen ion.

Question 24

What three details must be shown in the first step of an electrophilic addition mechanism?

Answer 24

Delta charges on the electrophile (eg HX)
A curly arrow from the C=C double bond to the delta + charge
A curly arrow from the H-X bond to the X atom

Question 25

Describe the second step of electrophilic addition between an alkene and a hydrogen halide.

Answer 25

The lone electron pair on the halogen atom is electrostatically attracted to the carbocation left over from the C=C double bond. They form a covalent bond together to give a complete haloalkane.

Question 26

What three three must be shown in the second step of an electrophilic addition mechanism?

Answer 26

Charges on both the halogen ion and the carbo-cation
A lone electron pair on the halogen atom
A curly arrow from the lone electron pair to the + charge on the carbocation

Question 27

How can a halogen molecule act as an electrophile?

Answer 27

Proximity to the negative charge of a C=C pi bond can induce polarity in a halogen molecule, allowing it to act as an electrophile.

Question 28

What is Markownikoff’s rule?

Answer 28

Markownikoff’s rule states that when a hydrogen halide is added to an unsymmetrical alkene, the most likely product is the one which involves the formation of the most stable carbocation- ie, the H atom adds to the carbon which already has the most H atoms attached to it.

Question 29

What is the major product in an alkene halogenation reaction.

Answer 29

The product where the H atom adds on to the carbon which already carries the most H atoms.

Question 30

What is a tertiary carbocation?

Answer 30

A C+ ion attached to three other C atoms- the most stable carbocation.

Question 31

What is a secondary carbocation?

Answer 31

A C+ ion attached to two other C atoms

Question 32

What is a primary carbocation?

Answer 32

A C+ ion attached to one C atom- the least stable carbocation.