Hydrocarbons I:Alkanes

IntroductionAlkanes are • the simplest organic molecules, consisting of only

carbon and hydrogen and with only single bonds between carbon atoms.

• are used as the basis for naming the majority of organic compounds (their nomenclature).

• have the general formula CnH2n+2.• are the simplest and least reactive hydrocarbon

species containing only carbons and hydrogens. • principal constituent of gasoline and lubricating oils,

are extensively employed in organic chemistry• lack of unsaturation, contains no double or triple

bonds, which are highly reactive in organic chemistry.

Structure classification

Saturated hydrocarbons can be:• linear (general formula CnH 2n + 2)

wherein the carbon atoms are joined in a snake-like structure

• branched (general formula CnH2n + 2, n > 3) wherein the carbon backbone splits off in one or more directions

• cyclic (general formula CnH2n, n > 2) wherein the carbon backbone is linked so as to form a loop.

Linear: Methane• simplest alkane• Methane, (CH4, one carbon bonded to four hydrogens) is • the simplest organic molecule. It is a gas at standard

temperature and pressure (STP).• The true three-dimensional form of methane does not have

any 90 degree angles between bonded hydrogens. The bonds point to the four corners of a tetrahedron, forming cos-1(-1/3) ≈ 109.5 degree bond angles.

Linear: Ethane

• two carbons singly bonded to each other with six hydrogens is called ethane• second simplest hydrocarbon molecule. It can be thoughtof as two methane molecules attached to each other, but with two fewer hydrogen atoms.

Drawing alkanes• When writing out the alkane structures, you

can use different levels of shorthand depending on the needs at hand in hand. For example, pentane can be written out. Its formula is C5H12.

o CH3–CH2–CH2–CH2–CH3,o CH3(CH2)3CH3,

Line drawing shorthand• Although non-cyclic alkanes are called straight-chain

alkanes they are technically made of kinked chains. • reflected in the line-drawing method. Each ending point

and bend in the line represents one carbon atom and each short line represents one single carbon-carbon bond.

• Every carbon is assumed to be surrounded with a maximum number of hydrogen atoms unless shown otherwise.

• Propane butane pentane hexane heptane

Conformations• Structural formula and the bond angles are

not usually sufficient to completely describe the geometry of a molecule.

• There is a further degree of freedom for each carbon – carbon bond: the torsion angle between the atoms or groups bound to the atoms at each end of the bond.

• The spatial arrangement described by the torsion angles of the molecule is known as its conformation.

Conformations

Conformation: Newman Projections

• Newman projections are drawings used to represent different positions of parts of molecules relative to each other in space. Remember that single bonds can rotate in space if not impeded. Newman projections represent different positions of rotating molecule parts.

• Conformers interconvert readily, normally thousands of times a second as parts of molecules spin.

• In the following drawings, methyl groups are on the front and back ends of the molecule and a circle represents all that lies between.

Conformation: Newman Projections

Conformation: Newman Projections

Conformation: Steric Effects

• Steric effects have to do with size. Two bulky objects run into each other and invade each others space. If we replace one or more hydrogen atoms on the above Newman projections with a methyl or other group, the potential energy goes up especially for the eclipsed conformations

• Lets look at a Newman projection of butane as it rotates counterclockwise around its axes.

Anti Eclipsed Gauche Eclipsed

Conformation: Steric Effects

Alkanes with unbranched carbon chains are simply named by the number of carbons in the chain. The first four members of the series (in terms of number of carbon atoms) are named as follows:

CH4 = methane = one hydrogen-saturated carbonC2H6 = ethane = two hydrogen-saturated carbonsC3H8 = propane = three hydrogen-saturated carbonsC4H10 = butane = four hydrogen-saturated carbons

Alkanes with five or more carbon atoms are named by adding the suffix -ane to the appropriate numerical multiplier, except the terminal -a is removed from the basic numerical term. Hence, C5H12 is called pentane, C6H14 is called hexane, C7H16

is called heptane and so forth.

Number of hydrogens to carbons

• This equation describes the relationship between the number of hydrogen and carbon atoms in alkanes:

• H = 2C + 2 where "C" and "H" are used to represent the number of carbon and hydrogen atoms present in one molecule. If C = 2, then H = 6.

• Many textbooks put this in the following format:

• CnH2n+2 where "Cn" and "H2n+2" represent the number of carbon and hydrogen atoms present in one molecule. If Cn = 3, then H2n+2 = 2(3) + 2 = 8.

• Low melting point• Low boiling point

o almost a linear relationship with the size(molecular weight) of the moleculeo rises 20–30 °C for each carbon added to the chaino A straight-chain alkane will have a boiling point

higher than a branched-chain alkane due to thegreater surface area in contact between adjacent molecules

o cycloalkanes tend to have higher boiling points than their linear counterparts

• Are colourless and odourless• Are not very reactive when compared with

other chemical species.• Increase in the number of carbons =

increase in melting point and boiling point

Chemical properties

• All alkanes react with oxygen in a combustion reaction, although they become increasingly difficult to ignite as the number of carbon atoms increases.• In the absence of sufficient oxygen, carbon

monoxide or even soot can be formed

Chemical properties

Isomers of Alkane

• Alkanes with more than three carbon atomscan be arranged in various different ways,forming structural isomers.

• The simplest isomer of an alkane is the one in which the carbon atoms are arranged in a single chain with no branches. This isomer is sometimes called the n-isomer

(n for "normal", although it is not necessarily the most common).

• The number of possible isomers increasesrapidly with the number of carbon atoms.

Examples

C1: methane only C2: ethane only C3: propane only C4: 2 isomers: n-butane and isobutane C5: 3 isomers: pentane, isopentane, and neopentane

Examples

C6: 5 isomers: hexane, 2-methylpentane, 3-methylpentane, 2,2-dimethylbutane, and 2,3-dimethylbutane C12: 355 isomers C32: 27,711,253,769 isomers C60: 22,158,734,535,770,411,074,184 isomers, many of which are not stable.

Examples

Examples