Alkenes

Question 1

What are alkenes?

Answer 1

A homologous series of unsaturated hydrocarbons.

Each alkene contains at least on C=C double bond.

Question 2

What is the general formula for the homologous series of alkenes?

Answer 2

CnH2n

Question 3

What type of bonds do single covalent bonds such as C-C or C-H consist of?

Answer 3

Sigma bond.

Question 4

What type of bonds do double bonds such as C=C consist of?

Answer 4

One sigma and one pi bond.

Question 5

Each carbon atom in ethene does what with its electrons?

Answer 5

It uses three of its four outer electrons to form three sigma bonds, two to hydrogen atoms and one to the other carbon atom.
The fourth electron on each carbon atom is found in a p orbital.

Question 6

What does overlap of two p orbitals on neighbouring carbon atoms produce?

Answer 6

A pi bond.

Question 7

What are compounds which contain pi bonds called?

Answer 7

Unsaturated compounds- indicates that the compound will combine by addition reactions with hydrogen or other chemicals, losing its multiple bonds.

Question 8

What are saturated compounds?

Answer 8

Contain only single carbon-carbon bonds.

Question 9

What does the characteristic reaction of an alkene involve?

Answer 9

A simple molecule (such as hydrogen, water or bromine) joining across the double bond to form a single product.
These addition reactions are the second of the important types of organic reactions that you will meet.

Question 10

What does adding hydrogen do to an unsaturated alkene?

Answer 10

Converts it into a saturated alkane.

Question 11

How is hydrogen addition carried out?

Answer 11

Hydrogen gas and a gaseous alkene are passed over a finely divided nickel catalyst supported on inert material.

Question 12

How is halogen addition carried out?

Answer 12

When an alkene such as propene is bubbled through a solution of bromine at room temperature, the bromine solution is rapidly decolourised from its characteristic orange colour.
Unlike free-radical substitution on an alkane, this reaction does not require UV light and will occur in total darkness .
The bromine joins to the propene to form 1,2- dibromopropane.

Question 13

Test and observation if an alkene is present?

Answer 13

Shake alkene with bromine water.

Orange bromine water is decolourised.

Question 14

Addition of hydrogen halides.

Answer 14

If ethene gas is bubbled through concentrated hcl at room temperature, chlorethane is produced.

Question 15

Addition of steam.

Answer 15

This is a route to making alcohols.
Industrially, steam and a gaseous alkene are passed over a solid catalyst.
A temperature of 330 degrees C and a pressure of 6 MPa are used in the presence of phosphoric acid, H3PO4, catalyst.

Question 16

What is the mechanism of addition (bromine and ethene)?

Answer 16

Although bromine and ethene are non-polar reagents, they react quickly at room temperature.
The bromine molecule becomes polarised when close to a region of negative charge (high electron density) such as the ethene pi bond.
The pi bonds then breaks, with its electron pair forming a new covalent bond to the bromine atom, which carries a partial positive charge.
At the same time, the bromine molecule undergoes heterolytic fission.
Heterolytic fission involves both electrons in the bond moving to the same atom.
This produces a bromide ion and a positively charged carbon atom (a carbocation).
This is highly reactive and the bromide ion rapidly forms a second carbon-bromine covalent bond to give 1,2-dibromoethane.

Question 17

What is an electrophile?

Answer 17

A reactant that accepts a pair of electrons to form a new covalent bond.

Question 18

What happens to an alkene during polymerisation?

Answer 18

An alkene undergoes an addition reaction to itself. When the addition reaction starts