hydrocarbons l. scheffler ib chemistry 3-4. lincoln h.s. 1

Hydrocarbons

L. Scheffler

IB Chemistry 3-4.

Lincoln H.S.

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• Hydrocarbons are organic compounds that contain only hydrogen and carbon

• Types of hydrocarbons include Alkanes Alkenes Alkynes Aromatic

Hydrocarbons

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• Alkanes have only carbon to carbon single bonds

• Every carbon has four chemical bonds either to hydrogen or another carbon atom

Alkanes

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• Alkanes have only carbon to carbon single bonds.• Each time a carbon atom is added to the chain

there are also two hydrogen atoms added.

Alkane Structures

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• With carbon chains that are four carbon atoms or longer there are multiple ways to arrange the

carbon chains.

Alkane Structures

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• Compounds that have the same molecular formula but different structural formulas are called Structural Isomers. The carbon chain may be consecutive or branched

Alkane Structures

Straight chain

Straight chain

Branched chain

Double Branched chain

Branched chain

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• Alkenes have one (or more) carbon to carbon double bonds

• Since there are fewer hydrogen atoms in alkenes as a result of the double bond, alkenes are referred to as unsaturated.

• Alkanes on the other hand have the maximum number of hydrogen atoms. They are referred to as saturated.

Alkenes

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• Like alkanes, alkenes can have branched or consecutive chains. In the larger alkenes there are also multiple locations for the C=C. Hence multiple structural isomers are possible.

Alkene Structures

Branched chain

Straight chain. The double bond is between the first and second carbon

Straight chain. The double bond is between the second and third carbon 8

• Alkynes have one (or more) carbon to carbon triple bonds

• Since there are fewer hydrogen atoms in alkynes as a result of the triple bond, alkynes like alkenes are referred to as unsaturated.

Alkynes

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• Like alkanes and alkenes, alkynes can have branched or consecutive chains. In the larger alkenes there are also multiple locations for the C=C. Multiple structural isomers are possible. The branch cannot originate on one of the carbons making up the triple bond

Alkyne Structures

Branched chain. The triple bond can occur in one of the branches but branches cannot be attached to any carbon in the triple bond

Straight chain. The triple bond is between the first and second carbon

Straight chain. The triple bond is between the second and third carbon 10

• Hydrocarbons that exist in chains are known as aliphatic hydrocarbons

• The ends of a chain may be joined to form a ring structure.

• These compounds are known as cyclic structures

Ring Structures

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• A number of the smaller alkanes exist as cyclic structures including– Cyclopropane– Cyclobutane– Cyclopentane– Cyclohexane

Saturated Cyclic Hydrocarbons

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Cyclic alkanes have the general formula

CnH2n. The additional C-C bond results in the loss of two hydrogen atoms

Saturated Cyclic Hydrocarbons

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• A few cyclic hydrocarbons may contain C=C double bonds. Two of the most common are cyclopentene and cyclohexene shown in the diagram at the left

Unsaturated Cyclic Hydrocarbons –

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• Writing structural formulas for cyclic hydrocarbons can be tedious. These short form structures are commonly used.

• Each vertex represents a carbon atom and it is implied that there are enough H atoms on each vertex to make four bonds

Cyclic Hydrocarbons –Condensed structures

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• The benzene ring is a common structure in organic molecules

• It consists of 6 carbon atoms and 6 hydrogen atoms.

• One would predict that there should also be 3 C=C bonds in a benzene ring

Aromatic Structures

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• Further investigation reveals that the double bonds are not distinct in benzene. Rather it is a resonance hybrid.

• Either of these structures could be used to represent benzene.

Aromatic Structures

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• Research shows that there are no differences in the C to C bonds in benzene.

• The current view of benzene holds that there are 6 C-C single bonds and 3 pairs or 6 delocalized electrons

Aromatic Structures

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• The structure of benzene is shown as either of these two structures, or as a circle in a hexagon which depicts that the electrons are delocalized

Aromatic Structures

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• Aromatic hydrocarbons are not limited to a single ring

• The fused ring structure shown is Naphthalene

Fused Aromatic Structures

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Reactions of Hydrocarbons

• Hydrocarbons tend to be very unreactive compounds when compared to other organic molecules.

• Most hydrocarbons are flammable. They burn in the presence of oxygen to form carbon dioxide and water vapor.

• Examples:

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Reactions of Hydrocarbons

• Most Hydrocarbons undergo substitution reactions in the presence of ultraviolet light

• Examples:

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Reactions of Alkenes• Hydrocarbons that have –C=C- undergo addition

reactions.

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Petroleum• Crude oil is a mixture of hydrocarbons formed

over along period of time from the slow decay of plant and animal matter

• It is separated by distillation into a variety of fractions

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Petroleum• C

rude

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Gasoline• Gasoline is a mixture of hydrocarbons.• The grade of a gasoline is based on a system known

as an octane rating.• Isooctane is a major component in gasoline that

burns evenly. It has octane rating of 100• Heptane burns with small explosions and tends to

cause engines to “knock”• The octane rating is the percentage of isooctane in the gasoline mixture• For an example: Gasoline with an octane rating of 87% isooctane and 13 % heptane.

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Natural Gas

• Natural gas is about 85% methane

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Halogenoalkanes or Alkyl Halides

• Halogenoalkanes are compounds in which one or more hydrogen atoms in an alkane have been replaced by halogen atoms (fluorine, chlorine, bromine or iodine).

• Halogenoalkanes are commonly known as alkyl halides

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Halogenoalkanes or Alkyl Halides

• Depending on the location of the halogen atom, halogenoalkanes may be primary secondary or tertiary

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Boiling Points of Halogenoalkanes

• The boiling point depends on the halide

Cl < Br < I• The boiling

points increase as the chain length increases

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Solubility of Halogenoalkanes

• The halogenoalkanes are only very slightly soluble in water.

• The attractions between the halogenoalkane molecules (van der Waals dispersion and dipole-dipole interactions) are relatively strong

• Halogenoalkanes are only slightly polar and do not effectively break the hydrogen bonds between water molecules.

• Halogenoalkanes are soluble in non polar or less polar organic solvents such as alcohol, ether, and benzene .

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