Alkenes and polymers

Question 1

What is the structure of ethene?

Answer 1

Each carbon in the ethene molecule is surrounded by 3 regions of electron density. These regions repel each other by equal amounts so the resultant bond angles between each carbon atom is 120 degrees, a trigonal plainer, making ethene a flat plainer molecule.

Question 2

Why is ethene important?

Answer 2

Ethene is obtained from catalytic cracking of long hydrocarbons in crude oil. The most important use of ethene is its use in the polymer industry.

Question 3

How is the C=C bond formed?

Answer 3

Covalent bonds will be formed by the sharing of electrons between the orbitals in the carbon atom.

1. The C=C bond consists of 2 types of bonds; the sigma bond and the pi bond.
2. The sigma bond is formed by the end-on overlap of orbitals. Each carbon contributes 1 electron to the bonding pair of electrons. This is the best overlap and thus the bond is the strongest.
3. The pi bond consists of 2 halves formed by the side-on overlap of p-orbitals from both carbon atoms, one above the plane of the carbon atoms and one below. Each carbon contributes 1 electron to the bonding pair. This overlap is less good compared to the sigma bond so the pi bond is weaker.

Question 4

What are stereoisomers?

Answer 4

Compounds with the same structural formula but with a different arrangement of the atoms in space.

Question 5

What is E/Z isomerism?

Answer 5

A type of stereoisomerism in which different groups attached to each carbon of a C=C double bond may be arranged differently in space because of the restricted rotation of the C=C bond.

Question 6

How does E/Z isomerism arise?

Answer 6

Due to the restricted rotation of the C=C double bond; if each carbon in the C=C bond is attached to 2 different groups, then these groups may be arranged differently in space in order to form E/Z isomers.

Question 7

What is cis-trans isomerism?

Answer 7

A special type of E/Z isomerism in which there is a non-hydrogen group and a hydrogen group on each C of a C=C double bond: the cis isomer (Z isomer) has the H atoms on each carbon on the same side; the trans (E isomer) has H atoms on each carbon on different sides.

Question 8

What addition reactions do alkenes take part in?

Answer 8

1. Addition of hydrogen halide: Gaseous hydrogen halide is bubbled through liquid alkenes at room temperature to form halogenoalkanes.
2. Addition of steam (hydration of alkenes): Steam and gaseous alkenes are passed over concentrated phosphoric acid catalyst at high temperature and pressure to produce alcohols.
3. Addition of halogens: Gaseous alkenes are bubbled through aqueous halogens or liquid alkenes are mixed with aqueous halogens to form di-halogenoalkane.
4. Addition of hydrogen: Gaseous hydrogen and alkenes are passed over nickel catalyst at temperatures of 150 degrees Celsius to form alkanes.

Question 9

Addition reactions of unsymmetrical alkenes.

Answer 9

If the alkene is unsymmetrical across the the C=C bond and the molecule being added is also unsymmetrical, then a mixture of products will be formed.

Question 10

What is an electrophile and what is electrophilic addition?

Answer 10

An atom or group of atoms that is attracted to an electron-rich centre or atom, where it accepts a pair of electrons to form a new covalent bond. The process is called electrophilic addition.

Question 11

What is the mechanism of electrophilic addition?

Answer 11

1. Pair of electrons in the pi bond attracted to the slightly positive atom in the polar molecule.
2. Pi bond breaks and electron pair moves to form a new C-H bond with one of the carbons in the C=C bond, leaving the other carbon with a positive charge.
3. H-X bond breaks by heterolytic fission and both electrons go to X molecule, forming an anion.
4. Carbocation reacts with the anion, with a lone pair on the anion used to form a new C-X bond.

Question 12

How can non-polar molecules take part in electrophilic addition reactions?

Answer 12

The electrons in the C=C bond repel electrons in the non-polar molecule, creating an induced dipole across the non-polar molecule.

Question 13

What is addition polymerisation?

Answer 13

The process of unsaturated alkene molecules being added to a polymer chain individually to form a long, saturated molecular chain.

Question 14

How is margarine formed from the hydrogenation of vegetable oil?

Answer 14

Vegetable oil is usually liquid at room temperature because it contains a large proportion of unsaturated fats (fats containing lots of C=C bonds). In order to harden vegetable oil, many of these C=C bonds need to be broken. The process of hydrogenation is used to break these bonds. Hydrogen gas is passed through vegetable oil in the presence of nickel catalyst which breaks some of the C=C bonds by adding hydrogen atoms across them. The reduced number of C=C bonds means that the vegetable oil can harden at room temperature, forming margarine.

Question 15

What is percentage yield?

Answer 15

Percentage yield is the measure of how efficient a reaction is at producing products, or:
(Actual amount, in mol, of product)/(Theoretical amount, in mol, of product)*100.

Question 16

What is atom economy?

Answer 16

Atom economy is a measure of how good a reaction is at producing useful products, or:
(Molecular mass of the desired product)/(Sum of molecular masses of all products)*100.