14 - Alkenes

Question 1

Are alkenes saturated or unsaturated?

Answer 1

Unsaturated

Question 2

What is a p orbital?

Answer 2

A bond containing a single electron on each carbon

Question 3

What is a pi orbital?

Answer 3

The 2 p orbital overlap to form an orbital with a cloud of electron density above and below the bond which restricts rotation

Question 4

Why can’t an alkene rotate?

Answer 4

There is a p orbital on each carbon.
They overlap to form an orbital with a cloud of electron density above and below the single bond - called a pi bond

Question 5

How to decide which atom to look at when looking at EZ isomers

Answer 5

The atom with the highest atomic number

Question 6

When is it E and when is it Z?

Answer 6

E if it is top and bottom
Z if it is just top/bottom

Question 7

Are alkenes soluble in water?

Answer 7

No

Question 8

Why are alkenes more reactive than alkanes?

Answer 8

C=C bond is electron rich which can be ‘attacked’ by positive reagents.

Question 9

What is an electophile

Answer 9

An electron deficient atom, ion or molecule that takes part in an organic reaction my attacking areas of high electron density in another atom

Question 10

How many electrons are there in the C=C double bond?

Answer 10

4

Question 11

Steps for electrophilc addition reaction

Answer 11

1. Electrophile is attracted to the double bond
2. Electrophiles are positively charged and accept a pair of electrons from the double bond. Electrophile may be a positively charged ion or have a positively charged area
3. A positive ion (carbocation) is formed
4. Negatively charged ion forms a bond with the carbocation

Question 12

Steps for hydrogen halide reaction

Eg ethene and hydrogen bromide

Answer 12

1. Bromine is more electronegative than hydrogen so the hydrogen bromide molecule is polar (H∂+ — Br∂-)
2. The Electrophile is the H∂+ of the H∂+ — Br∂-
3. The H∂+ of HBr is attracted to the C=C bond because of the double bond’s high electron density
4. One of the pairs of electrons from the C=C forms a bond with the H∂+ to form a positive ion (carbocation) whilst the electrons in the H∂+ — Br∂- bond are drawn to the Br∂-
5. The bond in hydrogen bromide breaks heterolytically. Both electrons from the shared pair in the bond go to the bromine atom because it is more electronegative than hydrogen leaving a Br- ion
6. The Br- ion attaches to the + charged carbon of the carbocation forming a bond with one of its electron pairs

Question 13

Positive inductive effect

Answer 13

Described the tendency of some atoms or groups of atoms to release electrons via a covalent bond

Question 14

Why is 2-bromopropane formed more than 1-bromopropane?

Answer 14

2-bromopropane has more alkyl groups surrounding it, so it is more stable

Question 15

What is heterolytic fission?

Answer 15

When one of the atoms takes both of the shared pair of electrons

Question 16

What is homolytic fission

Answer 16

Each atom takes one of the shared pair of electrons