Hydrocarbons

Question 1

What is homolytic fission?

Answer 1

The breaking of a covalent bond with one of the bonded electrons going to each atom, forming two radicals.

Question 2

What is heterolytic fission?

Answer 2

The breaking of a covalent bond with both of the bonded electrons going to one of the atoms, forming a cation and an anion

Question 3

What is an addition reaction?

Answer 3

A reaction in which a reactant is added to an unsaturated molecule to make a saturated molecule

Question 4

What is a free radical substitution reaction?

Answer 4

one or more hydrogen atoms is removed from a saturated hydrocarbon and replaced by another atom

Question 5

What is an elimination reaction?

Answer 5

The removal of a small molecule from a saturated molecule to make an unsaturated molecule

Question 6

What is an electrophile?

Answer 6

An electron pair acceptor

(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)

Question 7

What is a nucleophile?

Answer 7

AN ELECTRON PAIR DONOR

An atom (or group of atoms) that is attracted to an electron- deficient centre or atom, where it donates a pair of electrons to form a new covalent bond.

Question 8

Describe the bonding in alkanes

Answer 8

Covalent bonds between C-H and C-C formed by the overlap of orbitals creating sigma bonds consisting of one electron from each bonding atom.

Carbon always forms 4 bonds with surrounding atoms while H only forms 1

Question 9

Describe the shape of alkanes (bond angle)

Answer 9

Each carbon is surrounded by 4 electron pairs which repel each other to form bond angles of 109.5°. Long chain alkanes do not have fixed shapes, they could be zigzagged or they could form a U shape.

Question 10

What is the effect of chain length on boiling point of alkanes

Answer 10

As the chain length increase, more London forces are present between molecules as there is more surface area contact. The more London forces require a larger amount of energy to overcome therefore the boiling point is higher on longer chained alkanes.

Question 11

What is the effect of branching on boiling point of alkanes?

Answer 11

Alkanes with more branches have less surface area contact between each other meaning that there are less London forces present, therefore requiring less energy to overcome, meaning alkanes with more branching have lower melting points.

Question 12

Why is UV light required when reacting halogens with alkanes?

Answer 12

When reacting a halogen with an Alkane to form a haloalkane, UV light must be present as it provides the molecules with sufficient activation energy for the reaction to take place.

Question 13

Describe the initiation stage of bromination

Answer 13

The halogen (Br2) is broken down into radicals with free electrons (Br• + Br•)

Question 14

Describe the propagation stage of bromination

Answer 14

Their are two stages to propagation:

Stage 1- Br• is used to form a •CH3 radical and a HBr molecule

Stage 2- a Br2 molecule and the •CH3 form CH3Br and Br•

The cycle then repeats, the Br• used in stage 1 is formed in stage 2

Question 15

Limitations of radical substitution

Answer 15

• Further substitution (CH3Br, CH2Br2, CHBr3 or CBr4)- Substitution on different points on the chain (isomers can be formed)

Question 16

What is a free radical?

Answer 16

A species with an unpaired electron

Question 17

What is electrophilic addition?

Answer 17

An addition reaction in which the first step is attack by electrophile on a region of high electron density.

Question 18

What is electrophilic substitution?

Answer 18

A type of substitution reaction in which an electrophile is attracted to an electron rich centre atom, where it accepts a pair of electrons to from a new covalent bond

Question 19

Describe the nature of the double bond in alkenes

Answer 19

A sigma bond is formed between the C-C and then two p orbitals from each Carbon overlap above and below the plane of the sigma bond forming a pi bond.

Question 20

Describe the shape of the double bond

Answer 20

Trigonal planar- there are three regions of electron density forming bond angles of 120°.

Question 21

How does E/Z isomerism occur?

Answer 21

Rotation around the double bond is limited, meaning alkenes with the same molecular formulas can have different arrangements. The molecule must have C=C bonds and have different groups bonded to each C=C carbon.

Question 22

Describe Cis-Trans Isomerism

Answer 22

Cis-trans isomers have the same criteria as E/Z but with one extra rule- one of the groups attached to the C=C carbons must be the same, like in But-2-ene.

Question 23

Describe the Cahn-Ingold-Prelog nomenclature

Answer 23

Each group attached to the C=C carbons is given a priority based on their atomic number. If both high priority are above or below the C=C carbons it’s a Z isomer, whereas if a high priority and a low priority are above or below the C=C carbons it’s an E isomers

Question 24

What are the conditions required for hydrogenation of an alkene, and what is formed?

Answer 24

Hydrogenation of alkenes requires a high temperature and a nickel catalyst, this forms the corresponding alkane.

Question 25

What are the conditions and products of halogenation of alkenes?

Answer 25

Halogenation of alkenes can occur rapidly at room temperature and a haloalkane is formed (e.g. 2-bromoethane)

Question 26

Describe the test for saturation in in an unknown solution

Answer 26

Halogenation of alkenes can be used to determine the presence of C=C bonds. When bromine water is added dropwise to a sample if the solution turns colourless from orange then alkenes were present, if the solution remains orange no C=C bonds are present.

Question 27

Addition of hydrogen halides to alkenes

Answer 27

Alkenes react with gaseous hydrogen halides and HCl(aq) and HBr(aq) at room temperature

Question 28

What are the conditions and products of hydration reactions of alkenes

Answer 28

Alkenes react with steam in the presence of a H3PO4(aq) catalyst and from the corresponding alcohol.

Question 29

Describe the But-2-ene + HBr mechanism

Answer 29

HBr becomes dipolar with H as the + pole as Br is more electronegative. The electron rich pi bond attracts the polar H which causes heterolytic fission between H and Br and the double bond to break which forms a carbocation. The Br- ion forms a bond with the carbocation and 2-bromobutane is formed.

Question 30

Describe the propene + bromine mechanism

Answer 30

When Bromine, a non polar molecule, approaches propene the pi interact with the Br-Br bonding electrons which forms an induced dipole. The pi bond is attracted the to induced positive Br which causes the double bond to break. Br bonds to one of the carbons which leaves a Br- ion and a carbocation, these then bond and 1,2 dibromopropene is formed.

Question 31

Markownikoff’s rule

Answer 31

Markownikoffs rule predicts the most common product of addition when an unsymmetrical alkene forms two isomers. This is in the form of a major and a minor product. The minor product is when the halogen bonds to a primary carbocation while the major product bonds the halogen to a secondary carbocation.

Question 32

Briefly describe the carbocation stability trend

Answer 32

Primary is less stable than secondary which is less stable than tertiary which is the most stable carbocation.

Question 33

What is addition polymerisation?

Answer 33

Unsaturated alkene molecules (monomers) form long chains saturated chains (polymers)

Question 34

Environmental concerns with polymers

Answer 34

Very unreactive so they do not biodegrade, manufactured from finite fissile fuels that are unsustainable and release greenhouses gases when they’re made. Dangerous to wildlife if irresponsibly disposed of.