Special Sub-Topic: Alkanes
|Alkanes are hydrocarbons, as they only contain the elements hydrogen and carbon. What is the name of the simplest alkane?|
Methane. Methane is the simplest alkane, consisting of a single carbon atom surrounded by 4 hydrogen atoms, giving it a chemical formula of CH4. It has a molecular mass of 16 and because of this low molecular mass, it exists as a gas at room temperature.
|As alkanes are a homologous series, what is the general formula for a linear alkane?|
Cn H2n+2. Wikipedia defines a homologous series as, 'a series of organic compounds with similar chemical properties, members of which differ by a constant relative molecular mass.' For alkanes, the mass of each one differs by 14. This is because of the general formula, Cn H2n+2. For example, in methane, where n=1, the formula CH4 is produced. In ethane, where n=2, the formula C2H6 is found.
Cn Hn exists for benzene, C6H6 but not for a homologous series.
Cn H2n exists for the homologous series of alkenes.
Cn H2n-2 exists for the homologous series of alkynes.
|Carbon is capable of catenation, which is the ability of an element to form covalent bonds to itself. Carbon can form single, double or triple bonds to other carbon atoms, but which of these is present in alkanes?|
Single. Covalent bonding takes place when two atoms share an electron with each other. In alkanes, each carbon atom bonded to another provides one electron for a single bond.
Carbon-carbon double bonds exist in alkenes, as both carbon atoms share 2 electrons (but most still contain single bonds as well).
Carbon-carbon triple bonds exist in alkynes, where each carbon atom share 3 electrons with the other. (but most still contain single bonds as well).
Of course, organic chemistry is a huge area and many molecules contain all different types of bonds.
|How many carbon atoms does heptane contain?|
Seven & 7. The prefix hep- means it has seven carbon atoms in the longest chain, and as heptane is a linear alkane (all the carbons in one chain, not branched off), so it must have 7 carbon atoms.
The suffix, -ane, indicates that it is an alkane.
|What is formed when an alkane undergoes complete combustion?|
Carbon dioxide and water. Complete combustion is where there are sufficient levels of oxygen present. Combustion of any hydrocarbon will produce carbon dioxide and water.
Incomplete combustion is where not enough oxygen is present, forming carbon monoxide and water, or if there is a large lack of oxygen, carbon and water.
|In general terms, what is the name given to the product when an alkane reacts with a halogen?|
All of these (Halogenalkane, Haloalkane, Alkyl halogenide). The halogens are in Group VII of the periodic table and are fluorine, chlorine, bromine, and iodine. Chloroalkanes and bromoalkanes are the most common type of haloalkane found and studied.
|How many carbon atoms must be present in an alkane for the molecule to show structural isomerism?|
4. The correct answer is 4. Methane, ethane and propane all look the same as you can only arrange the atoms in one way.
Butane is the first alkane which can form an isomer, existing as straight chain butane, or 2-methylpropane.
|What is the general term used to describe an alkane in a ring structure?|
Cycloalkane. The smallest cycloalkane is propane, which looks like a triangle. They still have single carbon to carbon bonds with hydrogens attached, but each carbon is attached to another.
This gives cycloalkanes a general formula of Cn H2n, the same as an alkene.
|What is the trend in boiling points for straight chain alkanes?|
As the molecular mass increases, so does the boiling point. The first four alkanes are all gases (methane, ethane, propane and butane) at room temperature. The (straight chain) alkanes with a number of carbon atoms between 5 and 16 are liquids and solids begin to appear at C17H36.
The trend in boiling points is because there is very little polarity in alkanes, so the only force holding the atoms together is Van de Waals forces which are very weak. The more points of contact, the more Van de Waals forces present in a molecule. This is why as a molecule gets larger, (hence more points of contact), it needs a higher temperature to reach its boiling point.
|When an alkane exists in a ring structure, it has a higher boiling point than the corresponding straight chain alkane.|
T. This is because of more points of contact on a cycloalkane, so the boiling point is usually 10-20K higher than the corresponding alkane.
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