4.1.2 Alkanes

Question 1

what are alkanes the main components of and why is this the case

Answer 1

• natural gas and crude oil
• stability and lack of reactivity means that deposits of crude oil can remain on earth for years

Question 2

what types of molecules are alkanes

Answer 2

1) hydrocarbons (ONLY C and H)
2) saturated (ONLY C-C)

Question 3

what is the general formula of alkanes

Answer 3

CnH2n+2

Question 4

what bonds the carbons in an alkane

Answer 4

-sigma bonds (σ bonds)
- each carbon is joined to 4 other atoms by single covalent sigma bonds

Question 5

what is a sigma bond

Answer 5

• the result of the overlap of 2 orbitals, each with 1 electron each, positioned in line directly between the bonding atoms
• act as an axes around which the atoms can rotate freely
• form an electron dense region where - bond is

Question 6

what shape forms around each carbon in an alkane

Answer 6

• a tetrahedral shape
• 4 e- pairs in the 4 sigma bonds
• pairs repel as far as possible
• bond angles of 109.5 degrees

Question 7

why is the shape of alkanes not rigid

Answer 7

• the sigma bonds act as an axes around which the atoms can rotate freely (butane can be zig-zag or U shape

Question 8

why is fractional distillation possible in alkanes

Answer 8

• their boiling points vary as you increase the chain length

Question 9

what happens as chain length increases in alkanes in its effect on boiling points

Answer 9

1) more e-
2) larger SA so more SA points of contact between molecules
3) more london dispersion forces
4) require more energy to overcome greater forces

• boiling point INCREASES as chain length does

Question 10

what effect does branching have on alkane boiling points

Answer 10

• as you increase branching:
  1) fewer surface area points of contact
  2) weaker london dispersion forces
  3) branches get in way and stop molecules from getting too close too, decreasing forces more

-boiling point DECREASES as branching increases, (even though molar mass stays the same)

Question 11

why are alkanes not very reactive

Answer 11

1) the C-C and C-H sigma bonds are strong
2) C-C bonds are non-polar
3) C-H have similar EN, so can be considered non-polar as well

Question 12

why is the COMPLETE combustion of alkanes useful

Answer 12

• the hydrocarbons are readily available
• hydrocarbons are easy to transport
• no toxic products produced

Question 12

under what conditions does complete combustion take place

Answer 12

• with plentiful O2

Question 13

what is the equation for complete combustion of alkanes

Answer 13

CH4 + 2O2 ===> CO2 + 2H2O

• balance in order of C, H, O
• if uneven number of oxygens ever needed, can balance by using fractions

Question 14

under what conditions does oxygen undergo incomplete combustion

Answer 14

• not enough O2 present

Question 15

what happens during incomplete combustion and what products does this form

Answer 15

• C not fully oxidised ( H always is to produce water)
• forms CO, toxic gas
• even worse, can form C, solid soot

Question 16

when do halogens react with alkanes

Answer 16

• in the presence of UV sunlight
• called photolysis (bond breaking using UV light)

Question 17

what is the mechanism called in which halogens react with alkanes

Answer 17

• radical substitution

Question 18

what are the 3 steps of radical substitution, and explain what happens in each

Answer 18

1) INITIATION: the halogen molecule breaks into 2 radicals, with UV light ( a form of homolytic fission)
2) PROPOGATION: the halogen radical first reacts with the alkane molecule, and then the alkane radical formed reacts with the halogen molecule
3) TERMINATION: all of the radicals combine ( form desired product and others)

Question 19

what are the problems with radical substitution

Answer 19

• further substitution with halogen radicals can continue to take place, displacing more halogens past the product (chloroethane turns into dichloroethane)