alkanes Flashcards
why are alkanes considered saturated hydrocarbons?
alkanes contain only carbon and hydrogen atoms, and contain only single bonds
since each carbon has 4 bonds, alkanes only contain sp3 hybridised carbon atoms
are alkanes polar or non-polar?
non-polar. there is little difference in electronegativity between carbon and hydrogen, so the C-H bond has a very small dipole moment (non-polar bond). thus alkanes are non-polar and are held together mainly by weak dispersion forces
how does boiling point of alkanes change with number of carbon atoms in the chain?
the boiling point of alkanes increases regularly with increasing number of carbon atoms in the chain.
number of carbon atoms increases -> number of electrons in the molecule increases -> size & polarisability of electron cloud increases -> stronger dispersion forces between the molecules -> more energy required to overcome
how does boiling point of branched-chain alkanes and straight-chain alkanes (with the same number of carbon atoms) differ?
branched-chain alkanes have lower boiling points than straight chain alkanes with the same number of carbon atoms.
in branched-chain alkanes, there is smaller surface area of contact between branched-chain molecules, resulting in weaker dispersion forces between molecules and less energy required to overcome.
how does melting point of alkanes change with number of carbon atoms in the chain?
melting point generally increases, but the increase is not as regular as that observed for their boiling points as dispersion forces between molecules in a crystal depend not only on the size of the molecules but also on the arrangement of molecules in a solid crystal lattic
are alkanes soluble in non-polar solvents?
YES. alkanes are solbule in non-polar solvents as energy released from dispersion forces between alkane molecules and the non-polar solvent molecules is greater than or comparable to energy required to overcome dispersion forces between alkane molecules and the dispersion forces between non-polar solvent molecules.
examples of non-polar solvents: benzene, tetrachloromethane
are alkanes soluble in water?
NO. alkanes are insoluble in water as energy released from permanent dipole-induced dipole interactions and dispersion forces between alkane molecules and water molecules is lower than energy needed to overcome hydrogen bonding between water molecules and dispersion forces between alkane molecules
are liquid alkanes denser or less dense than water?
liquid alkanes are less dense than water and form an immiscible layer above water.
density of alkanes increase slightly with increasing number of carbon atoms. this is because increase in strength of dispersion forces causes alkane molecules to attract more closely together, resulting in slightly smaller volume of the liquid. since density = mass/vol, since Mr increases while vol decreases, density of the liquid alkane increases.
why does viscosity of alkanes increase with increasing number of carbon atoms present?
viscosity of alkanes increases with increasing number of carbon atoms present as dispersion forces between molecules increases. also because long molecules tend to tangle up with one another
what is free radical substitution?
alkanes react with halogens in the presence of ultraviolet light or heat (sources of energy) too form halogenoalkanes. this is known as free radical substitution
observations: (if chlorine is added) greenish-yellow Cl2 is decolourised
(if bromine is added) reddish-brown Br2 is decolourised
describe the mechanism of free radical subsitution of methane by chlorine
initiation: the Cl-Cl bond is homolytically broken to give chlorine free radicals. the energy required comes from the light absorbed or heat supplied
propagation: the highly reactive chlorine radical collides with a methane molecule and abstracts a hydrogen atom to produce HCl and a methyl radical. the methyl radical then reacts further with another molecule of chlorine to produce chloromethane and regenerate a new chlorine radical. (chlorine radical thus acts as a homogeneous catalyst, making the overall process a chain reaction as it is self-sustaining)
termination: 2 radicals collide and combine to form a stable product. when these reactions occur, radicals needed for the propagation steps are consumed, so the reaction cycle is broken and the chain reaction is terminated
why are C-H bond unaffected in the initiation step?
energy from UV light or heat is insufficient to break the strong C-H bond
in the propagation step, when methane reacts with the chlorine radical, why is HCl and .CH3 formed instead of CCl and .H?
enthalpy change of the reaction would be negative as the reaction is exothermic because energy is absorbed (UV light or heat) for the reaction to occur. enthalpy change where HCl and .CH3 is formed is negative, which corresponds with the above, whereas if CCl and .H is formed, the enthalpy change would be positive instead
why is a 3° radical more stable than the 1° radical?
the 3° radical is attached to 3 alkyl groups whereas the 1° radical is only attached to 1 alkyl group. the more alkyl groups attached to the carbon radical, the more stable the radical is, hence the more easily it will be formed.
what are the 2 factors that affect the ratio of products formed in a free radical substitution?
- the ratio of different types of hydrogen atoms (1°, 2°, 3°)
- stability of each carbon radical intermediate present
