Chapter 13 Alkenes

Question 1

What is the functional group of an alkane and how does it Form?

Answer 1

Alkanes are unsaturated hydrocarbons contains a C=C bond functional group.
This is comprised of a pi-bond (the sideways overlap of adjacent p-orbitals above and below the bonding C atoms) and a sigma-bond (overlap of orbitals directly between the bonding atoms
The pi-bond restricts rotation

Question 2

What shape does an alkane have and what are the bond angles around each C in the C=C bond?

Answer 2

The shape around each of the carbon atoms in the double bond is trigo al planar because:

• there are three regions of electron density around each of the carbon atoms
• the three regions repel each other as far apart as possible, so the bond angle around each carbon atom is 120
• all of the atoms are in the same plane

Question 3

What does the term ‘stereoisomers’ mean?

Answer 3

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

Question 4

What does the the term ‘E/Z isomerism’ mean?

Answer 4

An example of stereoisomerism, in terms of restricted rotation about a double bond and the requirement for two different groups to be attached to each carbon atom of the C=C group.

If both the heavier groups are on the same side (top or bottom) of their respective carbon atoms the isomer is Z but if they are on different sides of their respective carbon atoms the isomer is E

Question 5

What does the term ‘cis-trans isomerism’ mean?

Answer 5

A special case of E/Z isomerism in which Teo of he substitute the groups attached to each carbon of the C=C group are the same.

If they are on the same side of their respective carbon atoms (Z isomer) they are called a cis isomer. If they are on different sides of their respective carbon atoms (E isomer) they are cake,d a trans isomer.

Question 6

How are the CIP priority rules used to identify an E/Z isomer?

Answer 6

The Cahn-Ingold-Prelog priority rules state that the highest order (largest atomic number) of the two atoms attached to the one carbon in the C=C group.

If the two higher priority groups are on the same side of the double bond, the compound is a Z isomer.
If the two higher priority groups are diagonally places on the molecule (on different sides) the compound is an E isomer

Question 7

Are alkenes reactive? Why?

Answer 7

Alkenes are much more reactive than alkanes because of the pi-bond. Given that the electron density of the pi-bond is concentrated above and below the plane of the sigma-bond the electrons are more exposed and the pi-bond readily breaks and alkenes undergo addition reactions relatively easily.

Question 8

What happens in the hydrogenation of alkenes?

Answer 8

When an alkene is mixed with hydrogen and passed over a nickel catalyst and heated an addition reaction takes place to form an alkane

Question 9

What happens in the halogenation of alkenes? How can this be used to test saturation?

Answer 9

Alkenes undergo a rapid addition reaction with the halogens chlorine or bromine at room temperature.
This forms a dihaloalkane.

This can be used to test for saturation using bromine When bromine water (orange solution) is added dropwise to a sample of an alkene bromine adds across the double carbon bond. The orange colour disappears indicating the C=C bond but if the compound were already saturated the orange colour would remain.

ALKENES DECOLOURISE BROMINE WATER

Question 10

What happens when a hydrogen halide is added to an alkene?

Answer 10

Alkenes react with gaseous hydrogen halides at RTP to form haloalkanes

Question 11

What happens in the hydration of alkenes?

Answer 11

Alcohols form when alkenes react with steam and an acid (H3PO4) catalyst.
There are always two possible products depending on which carbon the OH group bonds to

Question 12

What is an electrophile?

Answer 12

An electron pair acceptor

Question 13

What is a nucleophile?

Answer 13

An electron pair donor

Question 14

Describe the mechanism for the reaction of ethene and hydrogen bromide.

Answer 14

1. Bromine is more electronegative than hydrogen so hydrogen bromide is polar and contains dipoles.
2. The electron pain in ethenes pi-bond is attracted to the particularly positive hydrogen atom, causing the double bond to break
3. A bond forms between the hydrogen atom of the H-Br molecule and a carbon atom that was a part of the double bond.
   4 the H-Br bond breaks heterolytically with the electron pair going to the bromine.
4. A bromide ion and a carbocation are former. A carbocations contains a positively charge carbon ion.
5. In the final step the Br- ion reacts with the carbocation to form the addition product

Question 15

What is Markownikoffs rule?

Answer 15

When a hydrogen halide reacts with an unsymmetrical alkene the hydrogen of the hydrogen halide attaches itself to the carbon atom of the alkene with the greater number of hydrogen atoms and the smaller number of carbon atoms already attached

Question 16

How does Markownikoffs rule create major and minor organic products? Use the reaction of propene and hydrogen bromide as an example.

Answer 16

Electrophilic addition occurs in two steps, the first step creates a carbocation. This carbocation could be primary or secondary. The primary carbocation is the carbon at the end of a carbon chain. In the secondary carbocation the positive charge is on a carbon atom with two carbon chains attached.

When propene reacts with hydrogen bromide, the major product is 2-bromopropane formed from the secondary carbocation. The yield of 1-bromopropane is much smaller.

Question 17

What is carbocation stability?

Answer 17

Carbocations are classified by the number of alkyl groups attached to the positively charge carbon atom. Tertiary carbocations (with three R/alkyl groups) are the most stable whilst primary carbocations are the least stable.

Carbocation stability is linked to the electron Donating ability of alkyl groups. Each alkyl group donates and pushes el trans toward the positive charge of the carbocation and the positive charge is spread slightly over the alkyl groups. The more alkyl groups attached to the carbocation the more it can spread the charge making the ion more stable

Question 18

Deduce the repeat unit of the polymer made from the monomer unit:
W    X
 |      |
C = C
 |      |
Y     Z

Answer 18

W   X
        |      |
-[- C - C -]- n
        |      |
       Y     Z

Question 19

What is addition polymerisation?

Answer 19

Unsaturated alkene molecules undergo addition polymerisation to produce long saturated chains containing no double bonds.

Many different polymers can be made, each with their own specific properties depending on the monomer used.

Question 20

How can we sustainably dispose of polymers?

Answer 20

• recycle plastics to be remade into new products
• combustion’s of the waste can be used for energy production.
• it can be used as an organic feedstock for the production of plastics and other organic chemicals
• disposal of halogenated plastics is dangerous as they cannot be burnt because they release toxic gases like HCl released from PVC. Therefore they must be recycled

Question 21

What are the environmental concerns of polymers?

Answer 21

They are readily available, cheap and convenient which means they are over used. However this poses a problem as they are not biodegradable meaning they fill landfills quickly and enter ecosystems killing plants and animals, especially marine life.

Question 22

What are biodegradable polymers? What are the benefits to the environment of these plastic alternatives?

Answer 22

They are able to be broken down by microorganisms into water, carbon dioxide and biological compounds. They are usually made from starch or cellulose.

The benefits are:

• they leave no visible or toxic residues
• they can be used as compost bags for food waste

Question 23

What are photodegradable polymers? What are their benefits to the environment?

Answer 23

The bonds between atoms are weakened by absorbing light meaning they degrade in the presence of sunlight.

This is good because they decompose into non-toxic biochemicals and do not use much space in landfills as they decompose relatively quickly