ALKANES Flashcards
explain BP trend down the homologous group for alkanes
- as size of alkanes ↑, e cloud size of each molecule ↑
-the e-cloud is ↑ polarisable, - more energy is req to overcome stronger idid int b/w larger alkane molecules
-BP ↑ down grp
compare BP of branches alkanes vs straight chain alkane
-branched mol have ↓ SA of contact b/w molecules compared to its straight chain isomer
-less energy is req to overcome less extensive idid int b/w branched molecules
-BP ↓
explain alkanes solubility in non polar solvents
-soluble in np solvent
-the energy rel from idid int formed b/w alkane and np solvent are sufficient to overcome the idid int within alk and np moelcules
compare stability & bond energy of C-C of alkane vs cycloalkane
unstable due to ring strain
-sp3 C atoms should have bond angle of 109.5 but C atoms are forced into a bond angle of 60 and 90
-easier to break C-C bonds as it is less stable, less exothermic B-E
compare alkanes density with that of water
-idid int formed b/w alk mol is weaker than H-bonding in H2O mol
-alk mol are pulled less closely to one another
-less alk mol per unit vol compared to H2O
explain solubility of alkanes in polar solvents/H2O
-insoluble in water
-no strong interactions formed b/w alk and H2O molecules
-hence insuff energy is released from idid b/w alk and H2O to overcome strong H-bonds b/w water molecules for solvation to occur
why are alkanes generally unreactive in nature/req high Ea for rxn to occur
-all C atoms are fully saturated
-C-H bonds are strong n non polar
-lack of e-rich and e-poor regions on alkane molecules
combustion eqns b/w alk and oxygen
CnH2n+2 + (3n+1)/2 O2 -> nCO2 + (n+1) H2O
why must UV light/high temp be required for FRS
to provide sufficient energy for the X2 molecule to undergo homolytic fission and form X radical
why is HX not formed in the termination step during FRS
H radical is highly unstable q
why is only a flash of UV light required for FRS
-only a small flash is required to generate the Cl rad due to the nature of FRS being a chain rxn since Cl rad is regenerated in the propagation step, making it a self sustaining cycle
-if XS UV light is used , may cause explosive reaction
Why does no reaction occur between I2 and alkanes
H-I bond is so weak that the overall enthalpy change of the propagation step is endothermic
since products are less energetically stable than the rxtants , no rxn occurs
Why tertiary RX formed in larger proportion than secondary/primary RX
tertiary radicals have the largest no. of e- donating alkyl grps -> disperse the e-def of radical to largest extent.
__ is formed from a primary radical while __ is formed from a secondary radical, hence forming__ is more favourable, increase rate of rxn, increasing the proportion of ___ formed
calc actual ratio of probability of products formed from FRS
theoretical ratio x relative rate of substitution
