o chem– fundamentals + isomerism + alkanes

Question 1

what is the empirical formula? (e.g. C3H6 => CH2)

Answer 1

it is the simplest formula that shows the ratio of all the different atoms in a molecule

Question 2

what is the molecular formula?

Answer 2

it is the actual number of atoms of each element in a molecule

Question 3

why is the structural formula useful?

Answer 3

it clearly shows how the atoms are bonded to each other

Question 4

give me two types of aliphatic (open chains) hydrocarbons

Answer 4

• straight-chain
• branched

Question 5

what class of compounds has the structure –C☰C–

Answer 5

alkyne

Question 6

what structure do arenes have?

Answer 6

benzene ring

Question 7

what structure do ethers have

Answer 7

R–O–R

Question 8

what’s the structural difference between ketones and aldehydes?

Answer 8

• ketone structure is R–C–H
  〢
  O
• aldehyde structure is R–C–R
  〢
  O

Question 9

is this an amide or an amine: –NH2

Answer 9

amine

Question 10

what class of compounds is this structure?R–C–NR2
〢
O

Answer 10

amide

Question 11

what defines a homologous series?

Answer 11

• same functional group
• each successive member increases by one –CH2 unit

Question 12

homologous series have the following characteristics:
- same _______ formula
- show a gradual change in _______ properties as molar mass increases
- have similar ________ properties

Answer 12

general, physical, chemical

Question 13

why is there resonance in benzene?

Answer 13

benzene has a delocalised π electron system

Question 14

benzene is stable because…

Answer 14

it is resonance stabilised

Question 15

what hybridisation does C in benzene undergo

Answer 15

sp2

Question 16

why is benzene so stable? (4)

Answer 16

• each C atom is sp2 hybridised
• each C atom contains one unhybridised p orbital that lies perpendicular to the plane of the benzene ring
• the p orbitals overlap sideways with one another -> continuous overlap of unhybridised p orbitals
• π e- can delocalise across benzene ring -> creates delocalised π e- cloud -> resonance

Question 17

what are the effects of resonance stability on benzene
1. ???
2. planar molecule
3. can undergo substitution rxns

Answer 17

identical C-C bond lengths

Question 18

why does benzene have identical C-C bond lengths?

Answer 18

• due to the partial double bond character of C–C bonds
• as a result of delocalisation of π e- across the ring

Question 19

why does benzene undergo substitution but not addition rxns?

Answer 19

• sub rxns preserve the resonance stability of benzene
• add rxns disrupt resonance stability -> energetically unfavourable

Question 20

what are the two reactions benzene can undergo?

Answer 20

• nitration
• reduction

Question 21

chain isomers have the same ______ formula and functional group, but different ________ skeleton

Answer 21

molecular, carbon

Question 22

positional isomers have different positions of ______________

Answer 22

functional group

Question 23

aldehydes are functional grp isomers with …

Answer 23

ketones

Question 24

carboxylic acids are functional grp isomers with…

Answer 24

esters

Question 25

alcohols are functional grp isomers with...

Answer 25

ethers

Question 26

what are stereoisomers?

Answer 26

molecules w same molecular formula and same structural formula, but **different spatial arrangement of its atoms**

Question 27

compounds that can have E/Z isomerism must: 1. have a covalent bond with ____________ 2. the two different grps bonded to ea C atom on the end of the restricted bond must be _________

Answer 27

restricted rotation, different

Question 28

in determining whether an isomer is E/Z, we need to determine which atom/molecule bonded directly to the carbon has higher ______ and thus higher _______

Answer 28

Mr, priority

Question 29

what are enantiomers?

Answer 29

stereoisomers that are **mirror images** of ea other and **non-superimposable** on ea other

Question 30

do enantiomers have identical physical, chemical and biological properties?

Answer 30

- yes for physical - yes for chemical except for reaction with other chiral molecules - no for bio as diff spatial arrangement makes them rxt w diff enzymes

Question 31

are alkanes polar?

Answer 31

no, they r non polar

Question 32

alkanes have _______ _____ boiling points

Answer 32

relatively low

Question 33

what is LDF in secondary school terms?

Answer 33

intermolecular FOA/ Van Der Waals' FOA

Question 34

what is the trend of boiling points down the homologous series of alkanes?

Answer 34

- electron cloud size ↑ - more polarisable - LDF ↑ -> energy required ↑ - ↑ b.p.

Question 35

do straight chain or branched alkanes have a higher boiling point? (idea: 'packing' of the molecules)

Answer 35

- straight chain - larger surface area - greater extent of LDF - requires more energy to break

Question 36

why are alkanes soluble in non-polar solvents? (hint: energy)

Answer 36

energy released from LDF btw alkanes + non-polar solvent releases enough energy to overcome LDF in alkanes and LDF in solvent

Question 37

why are alkanes insoluble in water? (hint: energy)

Answer 37

energy released from LDF btw alkanes + water is insufficient to overcome H bonding in water

Question 38

why are alkanes so unreactive? (3)

Answer 38

1. they are non-polar - do not attract any species for rxn 2. they are saturated molecules - no triple + double bonds to undergo addition 3. strong C-H and C-C bonds - rxns have high activation energy

Question 39

what are the products of complete combustion of alkanes?

Answer 39

CO2 and H2O

Question 40

what are the products of incomplete combustion of alkanes?

Answer 40

CO, unburnt hydrocarbons, soot (C)

Question 41

what is the reaction of alkanes with halogens called?

Answer 41

free radical substituition

Question 42

in free radical sub, what reagents and conditions are needed?

Answer 42

Cl2 (g) or Br2 (l), UV light/heat

Question 43

why are free radicals so reactive? (short)

Answer 43

free radicals have an unpaired electron

Question 44

UV light is required in free radical sub as it provides ________ for the ______________ of _______ to form Cl* radicals

Answer 44

energy, homolytic fisson, Cl-Cl bond

Question 45

how is homolytic fission of the Cl-Cl bonds achieved?

Answer 45

UV light provides energy for it

Question 46

the steps of free radical sub are: 1. initiation 2. ??? 3. termination

Answer 46

propagation

Question 47

in which step of FRS does the regeneration of the free radical take place?

Answer 47

propagation

Question 48

what are the products of FRS?

Answer 48

- HX - X halogenoalkane - trace amounts of alkane w double the no. of C atoms (X being the halogen)

Question 49

what happens during initiation of FRS? (1)

Answer 49

UV light provides energy for the homolytic fission of Cl-Cl to form Cl radicals

Question 50

what happens during propagation of FRS? (4)

Answer 50

- highly reactive Cl* radical reacts with alkane CH4, removing a H atom to produce HCl - as a result, another radical *CH3 is produced - methyl radical *CH3 reacts further w another Cl2 molecule to produce e final product -> CH3Cl - at the same time another Cl* is regenerated

Question 51

what happens during termination of FRS?

Answer 51

- when two radicals collide -> unpaired electrons pair up to form a stable product - this terminates the chain rxn - trace amt of ethane formed

Question 52

why is only a flash of UV light necessary to initiate the rxn?

Answer 52

- UV is only required for initiation - radicals can be regenerated -> allows for a chain rxn

Question 53

write out the mechanism w eqns ONLY for FRS on paper

Answer 53

-

Question 54

why is FRS NOT used to synthesise halogenalkanes?

Answer 54

it is difficult to control the - extent of substitution - position of subtitution

Question 55

what is a tertiary radical?

Answer 55

a radical with where the C atom w the unpaired electron is attached to three other alkyl grps

Question 56

why are tertiary radicals the most stable?

Answer 56

- has largest no. of e- donating alkyl groups - can disperse the e- deficiency of the radical to the largest extent

Question 57

how does the stability of radicals affect the actual ratio of products in FRS? (compare to expected ratio)

Answer 57

actual ratio may be different than expected

Question 58

how does the rate of FRS change down the halogen group?

Answer 58

it decreases

Question 59

why does the rate of FRS decrease down the halogen grp?

Answer 59

rate of FRS is dependent on the ∆H of the propagation step

Question 60

why is incomplete combustion harmful

Answer 60

- releases CO - CO binds irreversibly w haemoglobin in blood -> O2 deficiency

Question 61

what components of incomplete combustion can result in photochemical smog

Answer 61

- unburnt hydrocarbons rxt w O2, O3, oxides of nitrogen -> in e presence of sunlight

Question 62

NO2 can form _______ (environmentally harmful) and catalyses the formation of ____ to ____, forming _______ (also env harmful)

Answer 62

acid rain, SO2, SO3, acid rain