C10 Alkanes Flashcards
Describe alkanes. State the general formula of the two types of alkanes.
Alkanes are saturated hydrocarbons that contain only carbon-carbon and carbon-hydrogen single bonds and do not contain any double or triple bonds. (They are the simplest of the homologous series found in organic chemistry)
Two types of alkanes are open chain alkanes and cycloalkanes. Open chain alkanes have the general formula CnH2n+2 and successive members differ in composition by one CH2 group. Each C atom in an alkane molecule is sp^3 hybridised and has a tetrahedral molecular geometry around it. The bond angle about each C atom is about 109.5 degrees.
Cycloalkanes are alkanes that contain one or more rings of carbon atoms. The carbon atoms are arranged to form rings and they have the general formula CnH2n as the alkenes with only one C=C bond.
How is an alkyl group formed? Name the general formula and name of an alkyl group.
An alkyl group is formed by removing a hydrogen atom from an alkane such that it can be connected to the parent/main chain. The general formula of an alkyl group is CnH2n+1. To name an alkyl group, change the -ane ending of the alkane to -yl.
Explain the naming of alkanes.
Pg 3 to 4 of notes
methane ethane propane butane pentane hexane heptane octane nonane decane
Identify the longest carbon chain. This chain is called the parent chain.
Identify all of the substituents (groups appending from the parent chain).
Number the carbons of the parent chain from the end that gives the substituents the lowest numbers. When compairing a series of numbers, the series that is the “lowest” is the one which contains the lowest number at the occasion of the first difference. If two or more side chains are in equivalent positions, assign the lowest number to the one which will come first in the name.
If the same substituent occurs more than once, the location of each point on which the substituent occurs is given. In addition, the number of times the substituent group occurs is indicated by a prefix (di, tri, tetra, etc.).
If there are two or more different substituents they are listed in alphabetical order using the base name (ignore the prefixes). The only prefix whichisused when putting the substituents in alphabetical order isisoas in isopropyl or isobutyl. The prefixes sec- and tert- are not used in determining alphabetical order except when compared with each other.
If chains of equal length are competing for selection as the parent chain, then the choice goes in series to:
a) the chain which has the greatest number of side chains.
b) the chain whose substituents have the lowest- numbers.
c) the chain having the greatest number of carbon atoms in the smaller side chain.
d) the chain having the least branched side chains.
A cyclic (ring) hydrocarbon is designated by the prefixcyclo-which appears directly in front of the base name.
In summary, the name of the compound is written out with the substituents in alphabetical order followed by the base name (derived from the number of carbons in the parent chain). Commas are used between numbers and dashes are used between letters and numbers. There arenospaces in the name.
Describe the classification of carbon atoms.
Carbon atoms are classified as primary, denoted with 1 °, secondary, tertiary and quaternary depending on the number of C atoms to which they are bonded. A primary carbon atom is bonded to one other C atom, a secondary carbon atom is bonded to two other C atoms and so on.
Describe the classification of hydrogen atoms.
Hydrogen atoms are classified as primary, secondary, tertiary depending on the type of carbon atom to which they are bonded. A primary hydrogen atom is bonded to a C atom that is bonded to one other C atom. A secondary hydrogen atom is bonded to a C atom that is bonded to two other C atoms.
Note: There cannot be a quaternary hydrogen atom as the C atom cannot be bonded to 4 other C atoms and a H atom.
Describe the three types of isomerism that exist in alkanes.
Constitutional (structural) isomerism arises when there exist compounds with the same molecular formula but different structural formulae. From butane onwards, alkanes have constitutional isomers due to the branching of the hydrocarbon chains.
Stereoisomerism arises when there exist compounds with the same molecular formula and structural formula but differ in the way in which their atoms are arranged in space relative to one another.
One type of stereoisomerism is enantiomerism. Some branched alkanes display enantiomerism as they contain a chiral carbon centre and have no plane of symmetry, allowing them to be non-superimposable mirror images of each other.
Another type of stereoisomerism is cis-trans isomerism. In cycloalkanes, the ring structure restricts the free rotation of bonds in the ring, giving rise to cis-trans isomerism. (It is not possible for alkanes to have double bonds.)
Describe and explain the trend of boiling and melting points of alkanes with increasing number of carbon atoms.
The boiling and melting points of alkanes increase with increasing number of carbon aotms. Alkanes contain C-C and C-H bonds. Since the difference in electronegativity between the C and H atoms are negligible, the C-H bond and thus alkanes are essentially non-polar. The intermolecular forces of attraction between alkane molecules are relatively weak instantaneous dipole-induced dipole interactions. As the number of carbon atoms in alkanes increases, the number of electrons per alkane molecule increases and the size of the electron cloud of the alkane molecule increases. As a result, the ease of polarisation of the electron cloud increases leading to increasing strength of instantaneous dipole-induced dipole attractive forces which are overcome at increasingly higher temperatures.
Describe and explain the trend of boiling and melting points of alkanes with an increasing degree of branching.
The boiling and melting points of alkanes decrease with an increasing degree of branching. With branching, the molecule becomes more spherical (or more compact) and its surface area available for intermolecular interactions decreases. This reduces the extent of contact between neighbouring molecules, resulting in weaker instantaneous dispole-induced dipole forces that are overcome at a lower temperature.
Explain the solubility of alkanes in various solvents.
Alkanes are soluble in non-polar solvents such as benzene and are insoluble in water and other highly polar solvents. Some liquid alkanes (e.g. hexane) are used as non-polar solvents and they dissolve compounds of low polarity and do not dissolve compounds of high polarity.
Describe and explain the trend of the density of alkanes.
With the increasing molecular size of the alkanes, the density of alkanes increases. The density of alkanes tends to level off at about 0.8gcm^-3. Thus, all alkanes are less dense than water.
State the two ways in which alkanes can be prepared.
Alkanes can be prepared through:
1) Reduction of alkenes
2) Reduction of other unsaturated hydrocarbons.
Describe how alkanes can be prepared through the reduction of alkenes.
Reagents and conditions include H2 (g), Ni, heat OR H2 (g), Pt (Platinum), Pd (Palladium).
A mixture of alkene and hydrogen is passed over finely divided platinum, palladium or nickel catalyst. Since nickel is the least active of these catalysts, it requires an elevated temperature and pressure, whereas platinum and palladium function adequately at ordinary temperatures.
Describe how alkenes can be prepared through the reduction of other unsaturated hydrocarbons.
Alkynes can be reduced to other alkanes under suitable conditions.
For example, due to the triple bond of propyne, 2 moles of H2 (g) is required to reduce one mole of propyne to produce propane. Benzene can also be reduced to cyclohexane under suitable conditions.
Explain the general lack of reactivity of alkanes.
Alkanes are non-polar and consist of molecules that have no centres of electrical charge. They contain only non-polar C-C and essentially non-polar C-H bonds. They do not contain any region of high electron density and thus do not attract electrophilic reagents (electron-deficient reagents). They also do not contain any electron-deficient sites to attract nucleophilic reagents (electron-rich reagents). They are unable to attract charged species like H+, OH- or MnO4-. They are also unaffected by polar reagents such as acids, alkalis, dehydrating agents or aqueous oxidising agents. This often results in no effect, whether hot or cold and two immiscible layers can be observed
Alkanes have relatively strong C-C and C-H bonds which do not break under normal conditions.
State the two reactions that alkanes undergo.
Alkanes can undergo Combustion and Halogenation.
Describe the combustion of alkanes.
Alkanes burn readily in air or oxygen when ignited. In excess oxygen, alkanes burn with a non-sooty flame.
Complete combustion produces CO2 (g) and H2O (l).
Incomplete of alkanes leads to the formation of CO (g) and C (as soot) in addition to CO2 (g) and H2O (l).
On the other hand, unsaturated hydrocarbons burn with a sooty flame.
Why is alkane an important source of fuel?
The combustion reaction of alkanes is very exothermic, making alkane an important source of fuel.
