Alkanes for STPM Chemistry level are simply chemical compounds which are saturated and have only H and C atoms. By being saturated hydrocarbon compounds, they are bonded only by “single bonds“. They have the general formula of CnH2n+2. The simplest form of alkane is methane with just a single carbon atom with 4 hydrogen atoms attached to it through single bonds. The -ane suffix is used throughout the homologous series in Alkane, with varying prefixes that denotes the number of carbons. Meth-, eth-, prop- and but- are the first four prefixes being used.
This note, Part 1 of STPM Chemistry Form 6 notes by Berry Berry Easy on The Alkanes focusses on alkanes, some general definitions learnt in SPM level, selected physical properties and some pointers on the molecular structures of alkane. Although you do not need to have any detailed impression of alkanes learnt at SPM level, but it’ll be useful to recall what you have learnt previously.
[Tips: As Berry Readers would have learnt in SPM Chemistry, Alkanes can be classified under the same homologous series. This meant that all organic compounds within homologous series have similar general formula (with the exception of cyclic alkane) and possesses similar chemical properties due to the presense of the same chemical functional group. For alkane, the relative molecular mass increases by 14 for every increase in carbon. Do you know why? Do have a thought about it and answer it on your own to test your understanding from SPM Chemistry.]
STPM Chemistry Form 6 Note – The Alkanes (Part 1)
Saturated hydrocarbon – All alkanes are saturated hydrocarbons.
As recap to what you have learnt previously, the following are some general definitions which are used heavily throughout this chapters and other related chapters. They are:
- Molecular formula – describe the formula that gives the number of each kind of atom in a compound.
- Structural formula – describe the formula that gives the bonding arrangement of atoms in a molecule.
- Isomers – compound with the same molecular formula but different structural formula.
- Structural isomers – isomers that vary in the bonding attachments of atoms (5 types: skeletal, positional and functional isomers)
- Stereoisomers – isomers with same bonding attachments of atoms but different spatial orientation (2 types: geometric and optical/conformational)
General formula for noncyclic alkanes: CnH2n+2 (only carbon-carbon single bond)
General formula for cycloalkanes: CnH2n
Nomenclature of Branched-Chain Alkanes
- Find the longest carbon chain n the molecule. Use the Greek for the number of carbons followed by the suffix –ane.
- For cyclic compound, the ring is the base of the name.
- Name the attached chain (alkyl groups).
- Number the longest carbon chain from one end to the other that will gve the lowest number to the first substituent.
Nomenclature of Halogenated Hydrocarbons (Alkyl Halides)
- It follows the standard rules
- Halogens attached to a hydrocarbon chain are named by the prefixes fluoro- (F), chloro- (Cl), bromo- (Br) and iodo- (I).
Melting Point, Boiling Point and Molecular Weight
- Within the homologous series, M. P. and B. P. increase with increasing molecular weight.
- The larger the molecule, the more numerous are the sites for intermolecular attractions.
- The heavier the substance, the greater the energy needed to give the molecules sufficient force to break these intermolecular forces.
- M. P. trends are less regular than B. P. trends because melting also depends on the crystal lattice
Melting Point, Boiling Point and Molecular Structure
- B. P. decrease with the chain branching in hydrocarbons.
- Branching makes a molecule more compact and decreases the surface area (or total surface area).
- The smaller the surface area, the fewer the intermolecular attraction, therefore branched molecules have lower boiling point than unbranched hydrocarbon (same molecular weight).
- In contrast, compactness / molecular symmetry usually increase the M. P. of a compound because it fit more easily into a crystal lattice (more stable). Thus, a more stable crystal lattice structure requires a larger energy to disrupt it.
- Example isomers of C5H12: Pentane (M. P. = -160ºC / B. P. = 36ºC), 2-methylbutane (M. P. = -130ºC / B. P. = 28ºC) and 2,2-dimethylpropane (M. P. = -17ºC / B. P. = 9.5ºC). The melting points are higher for highly branched compounds than longer, straighter chain compounds.
- Another more dramatic example between isomers octane (M. P. = -57ºC / B. P. = 126ºC) and 2,2,3,3-tetramethylbutane (M. P. = 101ºC / B. P. = 107ºC).
Solubility and Density
- Alkanes are water insoluble.
- Alkanes are nonpolar carbon-carbon and carbon-hydrogen while water are polar solvent (polar O-H bonds).
- Hydrocarbons are less dense than water and will float on its surface.
This ends the first part of the series of notes on The Alkanes by Berry Berry Easy for STPM Chemistry Form 6 students. In the next part, you’ll learn about the general reactions of alkanes, which includes halogenation.