Alkanes/ Alkenes/ Alchols/ Haloalkanes

Question 1

Outline the bonding in alkanes

Answer 1

1) Covalent sigma bonds
Overlap of two orbitals, one from each bonding atom with two shared electrons

2) Each carbon atom has 4 sigma bonds

Question 2

Outline the shape of alkanes

Answer 2

1) Tetrahedral arrangement around each carbon atom, 109.5 º

Question 3

Melting/ Boiling points of alkanes

Answer 3

As relative molecular mass of alkanes increases, so does the boiling point

As chain length increases, molecules have larger surface area, so more surface contact possible between molecules. London forces between molecules increase, so more energy needed to overcome.

Boiling point decreases as degree of branching increases

Fewer surface points of contact between molecules of the branched alkanes, giving fewer London forces.

Question 4

Solubility of alkanes

Answer 4

Low polarity, so insoluble in polar solvents

Dissolve readily in each other

Question 5

Reactivity of alkanes

Answer 5

Unreactive:
1) C-C/ C-H bonds are strong
2) C-C bonds nonpolar
3) C-H have low polarity

Question 6

Physical properties of alkenes

Answer 6

Boiling points increase with increasing chain length, but lower in comparison to corresponding alkane

Question 7

Solubility of alkenes

Answer 7

Insoluble in water

Question 8

Bonding in alkenes

Answer 8

• Sigma/ pi bond

Pi bond is sideways overlap of two p orbitals on adjacent carbon atoms, each proving one electron

Density of pi bond concentrated above/ below line joining nuclei of bonding atoms

Locks 2 carbon atoms in position and prevents rotation around double bond.

Question 9

Shape of alkene bond

Answer 9

Trigonal planar
3 regions of electron density
Bond angle 120º

Question 10

Define stereoisomers

Answer 10

Same structural/ molecular formula but different arrangement in space

Question 11

E/Z isomerism

Answer 11

Stereoisomerism arises because restriction around double bond rotation, groups are fixed relative to each other.

Question 12

Cis/trans isomerism conditions

Answer 12

C=C double bond
Each carbon must be attached to 2 different groups, one of which must be hydrogen

Cis= H atoms on same side
Trans= H atoms diagonally opposite

Question 13

CIP nomenclature

Answer 13

Priority attached based on atomic number

High priority (larger numbers) on same sides= same side of molecule= Z isomer

Question 14

Reactivity of Alkenes

Answer 14

Alkenes are more reactive than alkanes because of the presence of the pi bond

Pi electrons are more exposed than the electrons in the sigma bond

Pi bond readily breaks and alkenes undergo addition reactions relatively easily

Question 15

Addition reaction of alkenes with hydrogen halides

Answer 15

• Alkenes + gaseous hydrogen halides at room temperature = haloalkanes

Or conc. HCl and HBr

Question 16

Hydration reactions with alkenes

Answer 16

Water as steam
Conc H2SO4/ H3PO4

Question 17

Electrophilic addition

Answer 17

C=C bond represents high electron density because of pi electrons

Attracts electrophiles

Question 18

Electrophiles

Answer 18

Atom/ group of atoms that is attracted to an electron rich centre and accept an electron pair

Question 19

Marknownikoff’s Rule

Answer 19

When hydrogen halide reacts with unsymmetrical alkene, hydrogen of hydrogen halide attaches to carbon with greater no. of hydrogen atoms

1º carbocation= positive charge at end of carbon chain (minor product)

2º carbocation= positive charge attached to carbon with two carbon chains attached (major product)

Question 20

Physical properties of alcohols

Answer 20

Miscible with water
High boiling points: hydrogen bonding

Hydrogen bond forms between lone pair of electrons (O) on OH group and hydrogen

Question 21

Alcohol solubility in water

Answer 21

Alcohols are soluble in water because hydrogen bonds form between alcohols and water molecules

Solubility decreases as hydrocarbon chain increases as influence of -OH group decreases.

Question 22

Reactivity of haloalkanes

Answer 22

Have carbon-halogen bond with their structure. Halogens are more electronegative than carbon, so carbon-halogen bond is polar.

Carbon atom attracts nucleophiles

Question 23

Nucleophile

Answer 23

Species that donate a lone pair of electrons

Question 24

Hydrolysis of haloalkanes

Answer 24

C-F is strongest carbon-halogen bond, C-I weakest.

Rate of hydrolysis increases as strength of carbon-halogen bond

Hence iodoalkanes react the fastest
Fluroalkanes unreactive