Chapter 12 alkanes

Question 1

What are the uses of alkanes

Answer 1

• main componets in natural gas and crude oil
• mainly used in fuels

Question 2

What is the general formula of alkanes

Answer 2

CnH2n+2

Question 3

Describe the bonding in alkanes

Answer 3

• alkaes are saturated hydrocarbons containing only carbon and hydrogen atoms joined together by single covalent bonds
• each carbon atom is joined by four other atoms in single bonds
• these are called sigma bonds
• each carbon in an alkane has four sigma bonds

Question 4

What is a sigma bond

Answer 4

• the result of the overlap of two orbitals one from each bonding atom
• ach overlapping orbital contain one electron so the sigma bond has two electrons shared between bonding atoms
• line directly between bonding atoms

Question 5

What are the shapes of alkanes and how are they determined

Answer 5

• each carbon is surrounded by four electron pairs in four sigma bonds
• repulsion between these electron pairs result in a tetrahedral arrangment
• each bond angle is approxmatley 109.5
• sigma bonds lallow the atoms to rotate freely

Question 6

what is the effect of chain length on boiling point

Answer 6

• as chain length increases the molecules have a larger surface area so more surface contact is possible between these molecules
• so more londo forced between molecule so more energy equired to overcome the forces

Question 7

Describe the effect of branching on boiling points of alkanes

Answer 7

• increased branching there are fewer surface points of contact between molecules so fewer london forces
• branches get in the way prevent the branched molecule getting as close together
• boiling point lowers as less energy to overcome fewer london forces

Question 8

Describe the reactivity of alkanes

Answer 8

• alkanes to not react with most common reagents
• lack reactivity a C-C and C-H bonds are stron
• C-C bonds are non polar
• the electronegativity of carbon and hydrogen is so similar that the C-H bond can be considered non polar

Question 9

Describe the combustion of alkane

Answer 9

• react with plentiful supply of oxyfen to produce carbon dioxide and water
• give ut heat and so are used as fuels

Question 10

Describe the incomplete combustion of alkanes

Answer 10

• in limited supply of oxygen
• each CH2 need an exctra 1.5 O2 for complete combustion
• can form toxic carbon monoxide, carbon as soot and water

Question 11

When can incomplete combustion occur

Answer 11

• closed space
• car engine
• faulty heating sytems
• inadquate ventilation

Question 12

Describe the reaction of alkanes with halogens

Answer 12

• prescence of sunligt alkanes react with halogens
• high energy ultraviolet radiation provides energy for reaction to tak eplact
• substitution reaction as hydrogen atom has been substitued with halogen atom

Question 13

What is the mecahnism for the bromination of alkanes

Answer 13

• radical substitution
• three stages initiation, propagation and termination

Question 14

Describe initiation of radical substitution of bromine and an alkane

Answer 14

• reaction stas when covalent bond in a bromine molecule is broken by homolytic fission
• each bromine atom takes one electron from the pair forming two highly reactive bromine radicals
• energy provided by UV radation

Question 15

Describe propagation of radical substitution of bromine and an alkane

Answer 15

• first step - bromine radical reacts with C-H bond in alkane
• forming a alkyl radical and a molecule of HBr
• second step - each methyl radical reacts with another bromine molecule forming the organic product bromoalkane with a new bromine radical
• new bromine radical reacts with another alkane in first propagation continue to cycle the reaction

Question 16

Describe termination of radical substitution of bromine and an alkane

Answer 16

• two radicals collide froming a molecules with all electron paired
• bromine radicals react to form bromine
• alkyl radicals react to form alkane
• alkyl and bromine radical react to form bromoalkane
• when radicals collide and react they are removed from reaction stopping the reaction

Question 17

Describe the limitations of radical substiution in bromine and alkane reaction

Answer 17

• further substiution - in second propagation step the brromoalkane can collide with a bromine radical substituting antoerh hydrgen atom and continue till all the hydrogen atoms have been substituted out
• subsitution at different positions in carbon chain - if carbon chain is longer we can get a mixture of isomers by substitution at different positions in the carbon chain and with further substitution more possibilities of isomers