alkenes Flashcards
how does boiling point increase with increasing number of carbon atoms in alkenes?
number of carbon atoms increases -> increase in number of electrons -> increase in size of electron cloud -> stronger dispersion forces between molecules -> more energy required to overcome -> boiling point increases
trend for melting point is not as clear as that of boiling point as melting point also depends on how molecules are packed in the solid lattice
why do cis isomers of alkenes have higher boiling points that trans isomers?
using 1,2-dichloroethene
- in the cis isomer, the polar (C-Cl) bond results in dipole moments that do not cancel out (since Cl is on the same side of double bond) -> produces an overall dipole moment for the molecule -> cis isomer is polar -> permanent dipole-permanent dipole interactions (on top of dispersion forces) exist between molecules
- in the trans isomer, the 2 dipole moments cancel out such that the overall dipole moment is zero -> trans isomer is non-polar and only dispersion forces exist between molecule
- so more energy is required to overcome the stronger forces of attraction between cis isomers, leading to a higher boiling point
this effect is less noticeable if there is no big difference in polarity between the carbon atom and the substituent attached, eg if the substituent is methyl (relatively non-polar) so the cis isomer is not much more polar than the trans isomer
why do trans isomers of alkenes have higher melting point that the cis isomer?
- the trans isomer packs better than the cis isomer due to high symmetry of the trans isomer. the U shape of the cis isomer doesn’t pack well compared to the straighter shape of the trans isomer
- better packing of trans isomer in solid state -> dispersion forces work more effectively in holding molecules together compared to the cis isomer -> more energy is required to overcome the forces of attraction between the trans isomer -> higher melting point
are alkenes soluble in water and non-polar solvents?
alkenes are NOT soluble in water. non-substituted alkenes can only form permanent dipole-induced dipole intermolecular forces with water -> much lower/weaker than (and insufficient to overcome) the hydrogen bonding between water molecules and dispersion forces between alkene molecules -> alkenes are insoluble in water
alkenes ARE soluble in non-polar solvents. non-substituted alkenes can form favourable dispersion forces with non-polar solvents -> greater than/comparable to dispersion forces between alkane molecules & between non-polar solvent molecules -> alkenes are soluble in non-polar solvents
why do density of alkenes increase with number of carbon atoms present?
more carbon atoms -> increase in strength of intermolecular dispersion forces -> alkene molecules attract more closely together -> slightly smaller volume of liquid + larger Mr. since density = mass/vol, when vol decrease and mass increase, density of the liquid increases with increasing number of carbon atoms
why does viscosity of alkenes increase with increasing number of carbon atoms present?
more carbon atoms -> strength of dispersion forces increase -> viscosity increases
straight chain alkenes have higher viscosity than branched alkenes bc straight chain alkenes may get ‘tangled’
why are tertiary carbocations more stable then secondary/primary carbocations?
teritary carbocations have more electron-donating alkyl groups which disperse the positive charge on the central carbon atoms, making the carbocation intermediate more stable, and hence more likely to be formed
if carbocations have electron withdrawing substituents like halogens, the positive charge on the central carbon atoms will be intensified and the carbocation will be destabilised
what is markovnikov’s rule?
markovnikov’s rule states that in the addition of HX (hydrogen halide) to an alkene, the H atom adds to the C atom of the double bond that holds the greater number of H atoms
more H atoms typically bc of more alkyl substituents
how will a racemic mixture be formed (when a hydrogen halide is added)?
the shape around the positively charged carbon atom is trigonal planar, so there is an equal probability of it being atacked from the top or the bottom by the halide ion, so both enantiomers will form in equimolar amounts, forming a racemic mixture.
must be enantiomers (have chiral centre)!
racemic mixture = no optical activity
what are conditions required for the electrophilic addition of halogens?
- chlorine gas/bromine gas OR chlorine/bromine dissolved in CCl4
- room temp
halogenoalkanes are formed
greenish-yellow chlorine/reddish-brown bromine would be decolourised
usually carried out in the absence of UV light to prevent free radical substitution
what are conditions required for electrophilic addition of water/steam?
industrial method:
- steam
- H3PO4 catalyst
- high temp & high pressure
lab method:
- concentrated H2SO4
- cold temp
- H2O
alcohols are formed
what are conditions required for reduction via catalytic hydrogenation?
H2 gas, nickel (Ni) catalyst, room temperature & high pressure
alkanes are formed
what are conditions required for mild oxidation?
KMnO4, dilute NaOH, cold
purple KMnO4 should be decolourised & a brown ppt of MnO2 should be formed
what are conditions required for oxidative cleavage?
KMnO4, dilute H2SO4, heat
purple KMnO4 is decolourised. effervescence may be observed if CO2 is evolved. CO2 is evolved when alkenes with terminal C=C bonds undergo oxidation
