Hydrocarbons
I really love this background guys…

Alkenes
• Hydrocarbons with one or more double bonds
• “Unsaturated”
– Have fewer than the maximum amount of hydrogens
• Have the general formula of C n
H
2n

Structure of Alkenes
• Unlike alkanes, alkenes cannot rotate freely about the double bond.
– The side-to-side overlap in the

-bond makes this impossible without breaking the

-bond.

Structures of Alkenes
• This creates geometric isomers , which differ from each other in the spatial arrangement of groups about the double bond.

Alkenes
• Draw C
4
H
8
• Structure of alkenes affects their properties

Nomenclature of Alkenes
• Alkenes end with “-ene”
• The chain is numbered so the double bond gets the smallest possible number.
• cis-Alkenes have the carbons in the chain on the same side of the molecule.
• trans-Alkenes have the carbons in the chain on opposite sides of the molecule.

Nomenclature of Alkenes
• If there is more than one double bond, use “di, tri, or
tetra” before the ending “-ene”
– …. Just like with carbon groups (dimethyl)
• Name this compound:
• 1,3- butadiene

Alkynes
• Hydrocarbons with one or more triple bonds
• “Unsaturated”
– Have fewer than the maximum amount of hydrogens
• Have the general formula of C n
H
2n-2

Alkynes
• The method for naming alkynes is analogous to the naming of alkenes.
• However, the suffix is -yne rather than -ene.
4-methyl-2-pentyne

Properties of Alkenes and Alkynes
• Pure alkenes and alkynes are colorless
• Low-molar-mass compounds are gases; higher molecular weight compounds are liquids and solids
• Burn readily in air to form CO
2
– Combustion reactions and H
2
0

Reactions of Alkenes and
Alkynes
• One reaction of alkenes is the addition reaction .
– In it, two atoms with the X-Y formula(like hydrogen, halogens, hydrogen halides, and water) add across the double bond.
– One

-bond and one

-bond are replaced by two therefore,

H is negative.

-bonds;

Reactions of Alkenes and
Alkynes
• Hydrogenation
– An addition reaction where the reagent being added is hydrogen (H
2
)

Reaction of Alkenes and
Alkynes
Mechanism of an Addition Reaction
• In the first step, the 
-bond breaks and the new
C —H bond and a cation form.
• In the second step, a new bond forms between the negative bromide ion and the positive carbon.

Reactions of Alkenes and
Alkynes
• 1860s Vladimir Markovnikov studied alkene addition reactions that created isomers
– Markovnikov’s Rule
• When a reagent (HX) adds to an unsymmetrical alkene, the hydrogen atom in the reagents will add to the carbon that already has the higher number of carbons

Reactions of Alkenes and
Alkynes

Aromatic Hydrocarbons
• Aromatic hydrocarbons are cyclic hydrocarbons that have some particular features.
– There is a p-orbital on each atom.
• The molecule is planar.
• They have significant, usually not unpleasant, odors

Aromatic Hydrocarbons
• Many aromatic hydrocarbons are known by their common names

Aromatic Hydrocarbons
• In aromatic compounds, unlike in alkenes and alkynes, each pair of

-electrons does not sit between two atoms.
• Rather, the electrons are delocalized ; this stabilizes aromatic compounds.
– Gives the molecule in stability… resonance stabilization

Aromatic Reactions
• Due to this stabilization, aromatic compounds do not undergo addition reactions; they undergo substitution .
• In substitution reactions, hydrogen is replaced by a substituent

Structure of Aromatic
Compounds
• Two substituents on a benzene ring could have three possible relationships:
– ortho-: On adjacent carbons.
– meta-: With one carbon between them.
– para-: On opposite sides of ring.

Practice Exercises
• How many straight-chain isomers are there of hexene, C
6
H
12
?
• Answer: five (1-hexene, cis-2-hexene, trans-2hexene, cis-3-hexene, trans-3-hexene)

Practice Exercises
• Name the following compounds:
• Answer: (a) 4-methyl-2-heptene
(b) 3-propyl-1-hexyne.