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Hydrocarbons
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Alkenes
• Hydrocarbons with one or more double bonds
• “Unsaturated”
– Have fewer than the maximum amount of hydrogens
• Have the general formula of CnH2n
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 C4H8
• 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
Physical Properties of
Alkenes
• Melting and boiling points increase as mass (# of carbons) increase
• Alkenes w/ fewer than 4 carbon atoms are colourless gases
• Alkenes w/ 5 or more carbon atoms are liquids and solids
• Alkenes are insoluble in water w/ other polar solvents; they are good
solvents for other non-polar molecules
Stereoisomers
• Geometric isomers
– Occur as a result of a C=C
– Cis : identical groups appear
on adjacent carbons on the same side of the double
bond
– Trans: if those groups appear on opposite sides
Alkynes
• Hydrocarbons with one or more triple bonds
• “Unsaturated”
– Have fewer than the maximum amount of hydrogens
• Have the general formula of CnH2n-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 CO2 and H20
– Combustion reactions
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 -bonds;
therefore, H is negative.
Reactions of Alkenes and
Alkynes
• Hydrogenation
– An addition reaction where the reagent being
added is hydrogen (H2)
– Essentially turns alkenes into alkanes, etc
Reactions of Alkenes and
Alkynes
• Hydration (addition of water)
– Alkenes and water, in the presences of a catalyst
(phosphoric acid, sulfuric acid, etc) form an
alcohol
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 hydrogens
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, C6H12?
• 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.