Organic chemistry- alkanes

Question 1

describe the bonding in alkanes

Answer 1

hydrogen and carbon atoms are covalently bonded. Each carbon atom has 4 sigma bonds.

Question 2

how is a sigma bond formed?

Answer 2

a sigma bond is a type of covalent bond. It is caused by the overlapping of two orbitals

Question 3

describe the shape of alkanes

Answer 3

four electron pairs in a sigma bond causes repulsion resulting in a tetrahedral arrangement where the bond angles are 109.5 degrees.

Question 4

how is crude oil separated into different fractions?

Answer 4

fractional distillation in a distillation tower. This is possible as the different alkanes have different boiling points.

Question 5

why does the length of the chain effect the boiling point?

Answer 5

the longer the chain, the higher the boiling point as the surface area is greater, more London forces will occur.

Question 6

what is the effect of branching on alkanes?

Answer 6

the greater the branching, the fewer the surface CONTACT so less London forces will occur

Question 7

why don’t alkanes react with most common reagents?

Answer 7

• sigma bonds are strong
• C-C bonds are non-polar
• the electronegativity of carbon and hydrogen are so similar that the C-H bond can be considered non-polar

Question 8

why are alkanes commonly used as fuel?

Answer 8

• combustion of alkanes give out heat
• easy to transport
• no toxic products

Question 9

complete combustion of alkanes forms?

Answer 9

carbon dioxide and water

Question 10

incomplete combustion of alkanes forms?

Answer 10

carbon monoxide and water. Further limited oxygen forms C and water.

Question 11

what is needed when alkanes react with halogens?

Answer 11

UV radiation. It provides energy for homolytic fission.

Question 12

interactions of alkanes and halogens are examples of substitution reactions and radical subsitution. What are the three stages for this mechanism?

Answer 12

initiation, propagation and termination

Question 13

what occurs during the initiation stage in free radical substitution?

Answer 13

the covalent bond of the halogen molecule is broken by homolytic fission, two highly reactive halogen radicals are formed.

Question 14

what is a radical?

Answer 14

a highly reactive species with an unpaired electron.

Question 15

what occurs during the propagation stage?

Answer 15

a chain reaction occurs through two main propagation steps. The two steps reoccur in a cycle.

Question 16

what occurs during the first propagation step? example: bromination of methane

Answer 16

the bromine radical will react with the C-H bond in the methane (CH4) to produce a methyl radical (CH3) and a molecule of hydrogen bromide (HBr).
CH4 + Br. -> .CH3 + HBr

Question 17

what occurs during the second propagation step?

Answer 17

the methyl radical will react with another bromine molecule, forming the organic product bromomethane, CH3Br, with a new bromine radical.
.CH3+Br2 -> CH3Br + Br.

Question 18

what occurs in the termination step?

Answer 18

two radicals collide forming a molecule with all electrons paired.

Question 19

what are the limitations of free radical substitution?

Answer 19

• further substitution occurs, creating a variety of products
• substitution can occur at different positions in the carbon chain

Question 20

when are sigma and pi bonds formed?

Answer 20

sigma bonds are present in the first bond. Pi bonds are present in any second or third bond.

Question 21

how is a pi bond formed?

Answer 21

by the unhybridized p-orbitals that exist above and below the sigma bond.

Question 22

what two stages are needed to change alkanes into alkenes?

Answer 22

fractional distillation and cracking

Question 23

What is the general formula for cycloalkanes?

Answer 23

CnH2n

Question 24

as carbo atoms in cycloalkanes increase, why does the boiling point increase?

Answer 24

more surface points of contact for London forces to occur which requires more energy to overcome.