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Organic Chemistry > 2. Alkanes > Flashcards

2. Alkanes Flashcards

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1
Q

what is a hydrocarbon?

A

a compound with only H and C atoms

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2
Q

general formula of alkane [without additional functional groups]

A

C₂H₂ₙ₊₂

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3
Q

are alkanes saturated or unsaturated?

A

saturated

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4
Q

alkanes cannot have cis-trans isomerism. T/F?

A

false. cyclohexanes CAN have cis trans isomerism due to ring structure restricting free rotation of bonds in the ring.

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5
Q

physical property of alkane: are they polar or non-polar? why?

A

non-polar. electronegativity between C and H negligible-> no dipole -> non-polar

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6
Q

physical property of alkane: boiling points increase/decrease with increasing number of carbons. why?

A

increase.
1. as no. of C atoms increase, no. of electrons in alkane molecule increase
2. size of electron cloud in molecule increases
3. ease of polarisation of electron cloud increases
4. strength of id-id attractive forces increases -> require more heat energy to overcome

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7
Q

Boiling points of isomeric alkanes ____ with increasing degree of branching.

A

Decrease.

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8
Q

Explain why boiling point of isomeric alkanes decrease with increasing degree of branching.

A
  1. With branching, molecule becomes more spherical/compact.
  2. Surface area available for intermolecular interactions decreases.
  3. This reduce extent of contact between neighbouring molecules.
  4. Weaker id-id attractive forces -> less heat energy required to overcome
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9
Q

Describe the melting point trend of alkanes

A

Increases as C atoms increases, but not a straight line, will be zigzag.

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10
Q

Describe the zigzag trend as observed for melting point of alkanes.

A

Alkanes with even number of carbon atoms are packed more closely than those w odd number of C atoms. Hence attractive forces between individual molecules are greater and melting point are higher.

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11
Q

solubility of alkanes

A

alkanes are non-polar->soluble in non-polar solvents but insoluble in polar solvents eg. water
solubility standard explanation
“soluble in organic solvents”
the energy released by the id-id between alkane & organic solvent molecules are sufficient to overcome the id-id between alkane & id-id between organic solvent molecules
“insoluble in water”
the energy released by the id-id between water and alkane molecules are insufficient to overcome the hydrogen bonding between water molecules & id-id between alkane molecules

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12
Q

trend of density of alkanes + compared to water’s density [2]

A
  1. increases with increasing molecular size, but plateaus at 0.8g/cm-3
  2. so all alkanes are less dense than water
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13
Q

how can alkanes be formed?

A

reduction of alkenes (catalytic hydrogenation of alkenes)

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14
Q

state reagents and conditions for reduction of alkenes [2]

A
  1. (H2 + Ni + heat) OR
  2. (H2 + Pt/Pd)

when using Nickel, need heat bc it is the least reactive catalyst

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15
Q

other than alkenes, what other kinds of compounds can be reduced? [2]

A

compounds that are unsaturated [aka double/triple bond]

eg. -CN, benzene, -C=O
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16
Q

are alkanes reactive? why or why not? [3]

A

alkanes are generally unreactive
1. non-polar, hence cannot attract charged species eg. H+, OH-
2. are saturated hydrocarbons, no double/triple bonds -> no regions of high electron density-> dont attract electrophilic reagents. also no e- deficient sites to attract nucleophiles
3. C-C & C-H bonds are strong and generally do not break under normal conditions

17
Q

state the effect of adding NaOH to alkanes

A

no effect, whether hot or cold

18
Q

state the effect of adding HCl to alkanes

A

no effect, whether hot or cold

19
Q

state the effect of adding conc H2SO4 to alkanes

recall: what is conc H2SO4 used for?

A

no effect

conc H2SO4 used to dehydrate alcohol to form alkenes

20
Q

state the effect of adding acidified KMnO4 to alkanes

recall: what is KMnO4 used for?

A

no decolourisation of purple KMnO4

KMnO4 used for oxidation

21
Q

state the effect of reacting air with alkanes

recall: what is air used for?

A

no effect when cold, burns when heated

air usually for combustion [high heat] of hydrocarbon

22
Q

state the effect of adding bromine to alkanes

recall: what kinds of reactions

A

no effect in the dark. bromine decolourised in presence of sunlight (UV)

free radical substitution

23
Q

state the 3 steps of FRS

A
  1. initiation: homolytic fission of Cl-Cl bond, forming Cl radicals in UV/sunlight
  2. propagation: Cl radical extracts H atom from CH4 to form HCl & CH3 methyl radical ; CH3 radical abstracts Cl atom from Cl2 molecule to form CH3Cl & Cl radical ; Steps 1 & 2 repeated to form a chain reaction
  3. termination: all the radicals form bonds w each other to form diff compounds
24
Q

How is CO formed? environmental consequence of CO

A

Incomplete combustion of fuel. Effect: Combines w Hb in blood and reduce blood capacity to transport oxygen

25
Q

environmental consequence of unburnt hydrocarbons

A

form photochemical smog in presence of sunlight-> irritates respiratory tract and results in chest pain and breathing problems

26
Q

How is nitrogen oxides formed? environmental consequence of their formation.

A

combination of nitrogen and oxygen in air at high temp in internal combustion engines

Effect: contribute to formation of acid rain ->
- makes farmlands unsuitable for cultivation
- corrode architecture
- negatively affect marine life [due to increased acidity of water]
- contributor of photochemical smog

27
Q

solution to reducing pollutants eg CO, NO2, unburnt hydrocarbons]

A

fit cars with catalytic converter, which converts these pollutants to CO2,N2 and water vapour.

28
Q

stability of radicals depends on?

A

number of alkyl groups bonded to C atom with unpaired electrons

29
Q

state whether there is bond formation or bond breaking in the following steps of FRS: Initiation

Recall what happens in initiation

A

bond breaking

30
Q

state whether there is bond formation or bond breaking in the following steps of FRS: Propagation

A

both formation and breaking

31
Q

state whether there is bond formation or bond breaking in the following steps of FRS: Termination

A

formation.
recall: all the radicals produced in initation and propagation bond with each other

32
Q

FRS can happen in the dark/room temp. T/F?

A

FALSE. need UV or light [aka energy to BREAK BONDS in initiation]

32
Q

FRS can happen in the dark/room temp. T/F?

A

FALSE. need UV or light [aka energy to BREAK BONDS in initiation]

33
Q

why is the synthesis of halogenoalkane from alkane through free radical substitution not preferred?

A
  1. difficult to control the extent of substitution
  2. formation of by-products
  3. difficult to control position of substitution

Organic Chemistry (9 decks)

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