Alkenes

Question 1

Define Alkene

Answer 1

Alkenes are unsaturated hydrocarbons containing at least one c=c double carbon bond.

Question 2

Describe alkene bonding (c=c)

Answer 2

Alkenes contain one double carbon bond which comprises of one π (pi) bond and one σ (sigma) bond. The sigma bond results from the overlap of two orbitals. The pi bond results from the sideways overlap of adjacet p orbitals.

Question 3

Why are alkene’s C=C bond rigid.

Answer 3

The pi bond prevents rotation about the double bond.

Question 4

Describe the shape around the double bond and why?

Answer 4

The shape around each carbon atom in the double bond is trigonal planar (120°). Since there are three regions of electron density around each carbon atom (in the C=C bond), the regions repel each other as far as possible .

Question 5

Define sterioisomers

Answer 5

sterioisomers are compounds with the same structural formula but with different arrangements of atoms in space.

Question 6

What two conditions must occur for E/Z sterioisomers?

Answer 6

• Compound must contain a C=C double carbon bond.
• Different groups must be attached to each carbon in the double bond.

Question 7

Draw the Z/cis isomer of But-2-ene

Answer 7

CH3 ‎ CH3
C = C
H ‎ ‎ ‎ ‎ H

Question 8

Define cis/trans isomers (3)

Answer 8

• Only occurs in compounds with C=C double carbon bonds.
• Different groups must be attaches to each carbon in the double bond
• One of the different groups attached to each double bonded carbon must be hydrogen.

Question 9

Draw the E/ trans isomer of But-2-ene

Answer 9

CH3 ‎ ‎ H
C = C
‎‎‎ ‎ ‎ H ‎‎ ‎ CH3

Question 10

Describe the reactivity of alkenes and why?

Answer 10

Alkenes have a high reactivity due to the π bond formed with the C=C double bond. Unlike the σ bond, the π bonds electrons are outside of the double bond and more exposed, allowing the π bond to easily break and react.

Question 11

What type of reaction do alkenes commonly undergo?

Answer 11

addition reactions

Question 12

Describe the process and enviroment for alkene hydrogenation

Answer 12

Alkene is mixed with hydrogen and passed over a nickel catalyst at 423K, forming an alkane.

Question 13

Describe the process and enviroment for alkene halogenation (Cl and Br)

Answer 13

Alkenes react with Chlorine and Bromine at room temperature, to form chloro-/ Bromo- alkanes.

Question 14

Describe how to test for an unsaturated alkene

Answer 14

Add bromine water drop wise to suspected alkene, bromine water should turn from orange to colourless.

Question 15

what two alkene addition reactions produce haloalkanes?

Answer 15

Halogenation and reactions with hydrogen halides.

Question 16

Describe the process and enviroment for alkene reaction with hydrogen halides

Answer 16

Alkenes react with gaseous hydrogen halides at room temperature to form haloalkanes. Or aqueous concentrated HCl and HBr.

Question 17

What is formed by the hydration of alkenes?

Answer 17

Alcohols

Question 18

Describe the process of hydrating alkenes

Answer 18

Alkenes react with steam in the presence of a phosphoric acid catalyst, to form an alcohol.

Question 19

Define electrophile

Answer 19

an electron pair acceptor

Question 20

What is the mechanism for alkene addition reactions?

Answer 20

electrophilic addition

Question 21

Describe the process of HBr reacting with an alkene

Answer 21

1. Bromine is more electronegative than hydrogen making a dipole H+ br-
2. The electron pair in the π bond is attracted to the partially positive hydrogen atom, breaking the C=C double bond.
3. A bond forms between the Hydrogen of H-Br and a carbon from the double bond, causing the H-Br bond to break by heterolytic fission.
4. This produces a Bromine ion and carbocation.
5. The Bromine ion reacts with the carbocation to form the product.

Question 22

What does Markownikoff’s rule state?

Answer 22

When a halogen halide reacts with an unsymetrical alkene the hydrogen from the halogen halide attaches itself to the carbon with the most hydrogens (out of the C=C bond).