Alkanes

Question 1

Physical properties of alkanes

Answer 1

• Melting and boiling points
  ○ At room temperature and atmospheric pressure, the first 4 alkanes (C1 to C4) are gases;
  The next 13 alkanes (C5 to C17) are liquids while longer alkanes (C18 and longer) are solids.
  ○ Alkanes have simple molecular structure and are non-polar.
  Hence, instantaneous dipole-induced dipole attractions exist between the alkane molecules.
  ○ The melting and boiling points of alkanes only depend on the strength of the id-id attractions between the molecules, which is dependent on:
  § Number of electrons in the molecule
  □ The greater the number of electrons, the bigger the size of the electron cloud, the more polarisable is the electron cloud, the stronger the intermolecular id-id attractions.
  § Surface area of contact of the molecule
  □ The greater the surface area of contact possible between the molecules, the greater the extent of id-id between the molecules.
  □ A straight-chain molecule will have a greater surface area of contact for intermolecular forces to develop as compared to a branched molecule of similar molecular size.
  • Solubility
    ○ Alkanes, being non-polar, are insoluble in polar solvents such as water as they cannot form hydrogen bonds with water.
    ○ Thus the energy evolved when id-id attractions is formed between the alkane and water molecules is insufficient to compensate the energy required to overcome the stronger hydrogen bonds between the water molecules.
    ○ However, they are soluble in non-polar solvents like tetrachloromethane, CCl4.
  • Density
    ○ As the size of the alkane increases, its molar mass increases and consequently its density increases.
    ○ However, the density tends towards 0.8 g cm-3.
    ○ Because alkanes are less dense than water and insoluble in water, alkanes in the liquid state floats on water.

[ Alkanes are non-polar because the C-H bond is considered non-polar as C and H have similar electronegativities ]

Question 2

Conditions of addition of hydrogen to alkenes

Answer 2

nickel, Ni
heat or platinum, Pt at room temperature

Question 3

Chemical properties of alkanes

Answer 3

• Unreactivity
  ○ Alkanes are organic compounds, which are very unreactive.
  ○ Alkanes are chemically inert to most reagents used in the organic chemistry.
  ○ In general, alkanes show a relatively low reactivity as:
  § They are non-polar and thus lack electron-deficient or electron-rich sites on the alkane molecules. They are therefore polar molecules such as water, as well as positive and negative ions such as OH- and H+.
  § In addition, the C-H and C-C bonds in alkanes are strong and they require a large amount of energy to break
  
  • Combustion reaction
    ○ Alkanes burn in excess oxygen to give carbon dioxide, water and heat. (general eqn for combustion of hydrocarbon applies)
    ○ When there is insufficient oxygen, incomplete combustion occurs and carbon monoxide or even soot (black solid) may be produced.
  • Free radical substitution with halogens

Question 4

Free radical substitution

Answer 4

reagent: limited amount of Cl2 or Br2
conditions: ultraviolet (UV) light or heat

- Key points to include when describing free radical substitution 
	○ Name of mechanism 
		§ Free radical substitution 
	○ Reaction conditions
		§ UV light
	○ Label the 3 stages
		§ Initiation, propagation, termination
	○ Construct balanced equations for each stage
		§ Initiation 
			□ Use of half arrows to denote movement of electrons in the initiation stage
		§ Propagation 
			□ Write enough equations until the required product (i.e. Multi-substituted alkane) is formed in the propagation stage
				® For each H atom that is substituted, there are 2 propagation steps: 
				Mono-substitution: 2 propagation steps 
				Di-substitution: 4 propagation steps 
		§ Termination: 
			□ At least 2 equations for termination stage
				® For each equation, use any 2 radicals formed in the mechanism to form the neutral product

Question 5

Homolytic VS Heterolytic

Answer 5

look at notes