alkanes

Question 1

Alkane bonds

Answer 1

Alkanes are saturated hydrocarbons

both the c-c and c-h bonds are made up of o-bond / sigma bond

the sigma bond forms between 2 C atoms by direct single overlap of orbitals directly between bonding atoms

allowing free rotation of sigma bond

MUST BE ABLE TO DRAW

Question 2

Boiling point of Alkanes explained

Answer 2

induced dipole dipole forces between molecules

chain length-
B-as C chain length increases so does boiling point

SC-more surface contact between molecules + more electrons

I-more IDD/ London interactions between molecules

E-need more energy to overcome

(use this one for cyclic compounds)

branching-
branched isomers have lower BP than unbranched

more branched= less surface contact between molecules

fewer IDD/ London interactions

need less energy to break weaker IDD forces between molecules

Question 3

shape of alkanes

Answer 3

tetrahedral

109.5⁰

4 bps around C atoms repel eacjotjer equally and get as far apart as possible

Question 4

Why are alkanes relatively unreactive?

Answer 4

C-C and C-H sigma binds are:

Non polar (almost- very small diff in electronegativity of C and H)

Strong

also have a high bond enthalpy giving them a low reactivity

Question 5

what reactions do alkanes undergo?

Answer 5

Combustion (complete and incomplete)

Halogenation

Question 6

Combustion

Answer 6

exothermic- releases heat energy to surroundings

alkanes are useful as fuels

complete-
plentiful supply of oxygen
burn completely
produce CO2 and H2O

incomplete-
limited supply of O2
burn incompletely
produce CO (toxic gas- prevent haemoglobin binding with O2, can be fatal) and H2O

Question 7

Radical substitution mechanism (halogenation)

Answer 7

Reagents- halogen and excess alkane

conditions-excess alkane, UV radiation or 300⁰C(provide energy to break bonds)

this is a substitution type reaction as the H atom in the alkane molecule is replaced by halogen atom

bonds break by homolytic fission to form radicals ( species with an unpaired electron)- Radical substitution

radicals shown using dot

Further substitution can occur to replace another H atom with a another halogen atom until all are replaces

mechanism

initiation-
reaction starts by breaking halogen bonds by homolytic fission to make 2 radicals
UV
Cl-Cl —-> 2 •Cl

Propagation-
a very reactive radicals attacks alkane molecule In chain reaction
1) Cl• + CH4 ——> •CH3 + HCl
2)•CH3 + Cl2 ——> CH3Cl + Cl•
the radicals should cancel out on either side
radical used up in 1st step and is them regenerated in 2nd step (act like catalyst)
radicals attack new alkane molecule and the reaction continues

termination-
the reaction ends when 2 radicals combine to form new molecules e.g.
Cl• + Cl• —> Cl2
•CH3 + Cl• —-> CH3Cl
•CH3 + •CH3 —-> C2H6

Question 8

limitations of radical substitution

Answer 8

leads to a formation of a mixture of products

further sub means mixture of haloalkanes are produced until all H atoms are replaced (chloromethane, dichloromethane, trichloromethane and tetrachloromethane)
this is why alkane is added in excess to block halogens from meeting eachother

may be reactions at different positions in C chain creating structural isomers (this is why we use molecular formulars(

Question 9

disadvantages of radical substitution

Answer 9

low % yield of desired Haloalkane due to structural isomers

separation of desired by fractional distillation is costly

to Avoid further subs an excess of alkanes is used which is costly