2. Alkanes Flashcards
what is a hydrocarbon?
a compound with only H and C atoms
general formula of alkane [without additional functional groups]
C₂H₂ₙ₊₂
are alkanes saturated or unsaturated?
saturated
alkanes cannot have cis-trans isomerism. T/F?
false. cyclohexanes CAN have cis trans isomerism due to ring structure restricting free rotation of bonds in the ring.
physical property of alkane: are they polar or non-polar? why?
non-polar. electronegativity between C and H negligible-> no dipole -> non-polar
physical property of alkane: boiling points increase/decrease with increasing number of carbons. why?
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
Boiling points of isomeric alkanes ____ with increasing degree of branching.
Decrease.
Explain why boiling point of isomeric alkanes decrease with increasing degree of branching.
- With branching, molecule becomes more spherical/compact.
- Surface area available for intermolecular interactions decreases.
- This reduce extent of contact between neighbouring molecules.
- Weaker id-id attractive forces -> less heat energy required to overcome
Describe the melting point trend of alkanes
Increases as C atoms increases, but not a straight line, will be zigzag.
Describe the zigzag trend as observed for melting point of alkanes.
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.
solubility of alkanes
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
trend of density of alkanes + compared to water’s density [2]
- increases with increasing molecular size, but plateaus at 0.8g/cm-3
- so all alkanes are less dense than water
how can alkanes be formed?
reduction of alkenes (catalytic hydrogenation of alkenes)
state reagents and conditions for reduction of alkenes [2]
- (H2 + Ni + heat) OR
- (H2 + Pt/Pd)
when using Nickel, need heat bc it is the least reactive catalyst
other than alkenes, what other kinds of compounds can be reduced? [2]
compounds that are unsaturated [aka double/triple bond]
are alkanes reactive? why or why not? [3]
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
state the effect of adding NaOH to alkanes
no effect, whether hot or cold
state the effect of adding HCl to alkanes
no effect, whether hot or cold
state the effect of adding conc H2SO4 to alkanes
recall: what is conc H2SO4 used for?
no effect
conc H2SO4 used to dehydrate alcohol to form alkenes
state the effect of adding acidified KMnO4 to alkanes
recall: what is KMnO4 used for?
no decolourisation of purple KMnO4
KMnO4 used for oxidation
state the effect of reacting air with alkanes
recall: what is air used for?
no effect when cold, burns when heated
air usually for combustion [high heat] of hydrocarbon
state the effect of adding bromine to alkanes
recall: what kinds of reactions
no effect in the dark. bromine decolourised in presence of sunlight (UV)
free radical substitution
state the 3 steps of FRS
- initiation: homolytic fission of Cl-Cl bond, forming Cl radicals in UV/sunlight
- 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
- termination: all the radicals form bonds w each other to form diff compounds
How is CO formed? environmental consequence of CO
Incomplete combustion of fuel. Effect: Combines w Hb in blood and reduce blood capacity to transport oxygen
environmental consequence of unburnt hydrocarbons
form photochemical smog in presence of sunlight-> irritates respiratory tract and results in chest pain and breathing problems
How is nitrogen oxides formed? environmental consequence of their formation.
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
solution to reducing pollutants eg CO, NO2, unburnt hydrocarbons]
fit cars with catalytic converter, which converts these pollutants to CO2,N2 and water vapour.
stability of radicals depends on?
number of alkyl groups bonded to C atom with unpaired electrons
state whether there is bond formation or bond breaking in the following steps of FRS: Initiation
Recall what happens in initiation
bond breaking
state whether there is bond formation or bond breaking in the following steps of FRS: Propagation
both formation and breaking
state whether there is bond formation or bond breaking in the following steps of FRS: Termination
formation.
recall: all the radicals produced in initation and propagation bond with each other
FRS can happen in the dark/room temp. T/F?
FALSE. need UV or light [aka energy to BREAK BONDS in initiation]
FRS can happen in the dark/room temp. T/F?
FALSE. need UV or light [aka energy to BREAK BONDS in initiation]
why is the synthesis of halogenoalkane from alkane through free radical substitution not preferred?
- difficult to control the extent of substitution
- formation of by-products
- difficult to control position of substitution
