9A. Alkanes + Cycloalkanes

Question 1

Alkanes

Answer 1

• saturated hydrocarbons are collectively called alkanes (or paraffins)
• simplest structure consists of carbon atoms in one continuous chain, known as ‘normal’ or ‘straight chain’ alkanes
• more complex structures have branches, known as ‘branched chain’ alkanes

Question 2

Two major sources

Answer 2

• natural gas
• petroleum
• natural gas + petroleum provide 90% of the organic raw materiels for the synthesis + manufacture of synthetics, detergents, plastics, dyes, drugs + many other products

Question 3

Natural Gas

Answer 3

• consists of 90 - 95% methane + 5 - 10% ethane + a mixture of other low boiling point alkanes, including propane, butane + 2-methylpropane

Question 4

Petroleum

Answer 4

• thick, viscous liquid mixture of thousands of compunds - mostly hydrocarbons - formed from the decomposition of marine plants + animals
• refined by fractional distillation whereby it is heated from 370^oC - 425^oC + separated into fractions, each containing a mixture of hydrocarbons that boil within a particular range
• fuel, grease + lubricants are products of petroleum

Question 5

General Formula

Answer 5

C_nH_2n+2

where ‘n’ is the number of carbon atoms in the molecule

• any two successive members of the alkane series differ by a single CH₂ group
• ‘homologous series’: in which one member differs from the next by the same repeating cluster of atoms

Question 6

Structure + Geometry

Answer 6

• overall shape of a hydrocarbon molecule is dependent on the shape of its carbon skeleton
• carbon has 4 single bonds arranged in a tetrahedral manner with bond angles of approximately 109.5^o
• free rotation exists about carbob-carbon single bonds, so even the simplest chain, ethane, has an infinite array of possible three-dimensional shapes, or conformations

Question 7

Conformation

Answer 7

• any three-dimensional arrangement of atoms in a molecule that results from rotation about a single bond
• at any one time there is an infinite array of possible conformations in a sample of a substance + each molecule constantly changes as it collides with other molecules or the walls of its container
• however, the majority of molecules will be in the most stable, fully extended conformation where each atom of the molecule is at its optimum distance from the others
• conversely, the fewest molecules will be in the most crowded conformation, where substituent groups are closest together

Question 8

Naming

Answer 8

1. the name for an alkane with an unbranched chain of carbon atoms consists of a prefix indicating the number of carbon atoms in the parent chain, and the suffix -ane
2. for branched chain alkanes, the longest chain of carbon atoms is the parent chain + its name becomes the root name
3. each substituent on the parent chain is named + allocated a number. The number is that of the carbon atom to which the substituent is bonded. A hyphen connects the number to the name
4. if there is one substituent, the parent chain is numbered from the end that gives the substituent the lower number
5. if the same substituent occurs more than once, the parent chain is numbered from the end where the substituent is first encountered. A prefix (di-, tri-, tetra-, penta-, hexa-, etc) is used to indicate the number of times the substituent occurs, + a comma separates the position numbers
6. if there are two or more different substituents, they are listed in alphabetical order + the chain is numbered from the end where a substituent is first encountered. If there are different substituents in equivalent positions on opposite ends of the chain, the substituent of lower alphabetical order will take the lower number
7. prefixes are not included when determining alphabetical order. The substituents are ordered first + then the prefixes are inserted
   - in the older system of common nomenclature, the total number of carbon atoms, regardless of their arrangement, determines the name
   - for alkanes beyond propane, the prefix ‘iso’ is used to indicate that one end of an otherwise unbranched chain terminates in a (CH₃)₂CH- group

Question 9

Physical properties

Answer 9

• most important is that alkanes are non-polar + therefore the only interactions between molecules are the very weak London dispersion forces
• melting + boiling points*
• boiling points are lower than that of almost any other type of compound with the same molecular weight
• generally, both boiling + melting points increase with increasing molecular weight
• alkanes containing 1 - 4 carbon atoms are gases at room temperature, those containing 5 - 17 carbon atoms are colourless liquids, + the higher molecular weight alkanes are white, waxy solids
• for any group of alkane constitutional isomers, the least branched isomer generally has the highest boiling point + the most branched isomer generally has the lowest boiling point due to the molecular shape of each isomer. As branching increases, the molecule becomes more compact + its surface area decreases so the London dispersion forces act over a smaller area. Therefore the attraction between molecules decreases, so the boiling point decreases
• solubility*
• because alkanes are non-polar they are not soluble in water
• they are soluble in each other + in other non-polar organic compounds
• density*
• all liquid and solid alkanes are less dense than water
• because they are insoluble in water, they will float on water

Question 10

Reactions

Answer 10

• most important chemical property of alkanes is that they are inert except for reactions with oxygen + the halogens
• oxidation, in the form of combustion, is the basis for which hydrocarbons are used as energy sources for heat + power
• the heat liberated when an alkane is oxidise to carbon dioxide + water is called its ‘heat of combustion’
• halogenation occurs when a hydrigen atom in the parent gydrocarbonis replaced with a halogen atom. The product is then known as a halogen derivative. When naming these compounds, the halogen is specified as ‘flouro’, ‘chloro’, ‘bromo’ or ‘iodo

Question 11

Alkyl Group

Answer 11

• a substituent groupderived from an alkane by removal of a hydrogen atom
• commonly represented by the symbol R-
• named by replacing -ane by -yl in the name of the parent alkane

Question 12

Cycloalkanes

Answer 12

• a saturated hydrocarbon in which the carbon atoms bond to form a ring
• ring sizes from 3 to more than 30 carbon atoms occur naturally
• 5 and 6 membered rings are especially abundant

Question 13

Naming Cycloalkanes

Answer 13

• the prefix ‘cyclo-‘ is added to the name of the corresponding open chain alkane
• the substituents are named+ only if there are two or more are they numbered
• the ring is numbered beginning with the substituent of lower alphabetical order

Question 14

Structure + Geometry of Cycloalkanes

Answer 14

• all cycloalkanes larger than cyclopropane favour non-planar or puckered conformations
• cis - trans isomerism is prevalent in cycloalkanes

cyclopentane

• the most stable conformation of cyclopentane is the envelope conformation, where four of the carbon atoms lie in a plane + the fifth is bent out of the plane like an envelope with its flap bent upward. All bond angles are approximately 109.5o

cyclohexane

• the most stable form of cyclohexane is the chair conformation in which all bond angles are approximately 109.5o
• the 12 C-H bonds are arranged in two different orientations. Six of them are axial bonds + six are equatorial bonds
• axial bonds are oriented parallel to an imaginary axis running from top to bottom through the centre of the chair
• three of the axial bonds point up and three point down, alternating up + down from one carbon to the next
• equatorial bonds are oriented approximately perpendicular to the imaginary axis + also alternate slightly up + slightly down from one carbon to the next
• if the axial bond on a carbon points upward, the equatorial bond on that carbon will point slightly downward
• converesly, if the axial bond points downward, the equatorial bond will point slightly upwards
• each equatorial bond is oriented parallel to two ring bonds on opposite sides of the ring
• the most stable form of cyclohexane occurs when the maximum number of substituent groups is equatorial