Chapter 12 alkanes Flashcards
1
Q
What are the uses of alkanes
A
- main componets in natural gas and crude oil
- mainly used in fuels
2
Q
What is the general formula of alkanes
A
CnH2n+2
3
Q
Describe the bonding in alkanes
A
- 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
4
Q
What is a sigma bond
A
- 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
5
Q
What are the shapes of alkanes and how are they determined
A
- 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
6
Q
what is the effect of chain length on boiling point
A
- 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
7
Q
Describe the effect of branching on boiling points of alkanes
A
- 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
8
Q
Describe the reactivity of alkanes
A
- 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
9
Q
Describe the combustion of alkane
A
- react with plentiful supply of oxyfen to produce carbon dioxide and water
- give ut heat and so are used as fuels
10
Q
Describe the incomplete combustion of alkanes
A
- 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
11
Q
When can incomplete combustion occur
A
- closed space
- car engine
- faulty heating sytems
- inadquate ventilation
12
Q
Describe the reaction of alkanes with halogens
A
- 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
13
Q
What is the mecahnism for the bromination of alkanes
A
- radical substitution
- three stages initiation, propagation and termination
14
Q
Describe initiation of radical substitution of bromine and an alkane
A
- 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
15
Q
Describe propagation of radical substitution of bromine and an alkane
A
- 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
