ALKENES…..

 STRUCTURE:
 Hydrocarbons can contain carbon bound to another
carbon by a double bond made of a sigma ___ bond and
a ___ bond. Compounds with double bonds are called
alkenes and are considered unsaturated compounds,
and an older systems calls olefins. The general structure
of an alkene in a chain is CnH2n, two hydrogens less
than an alkanes.
 The alkene carbon atoms are sp2 hybridized and all of
the single bonds are sp2 orbitals overlapped with orbitals
from hydrogen or carbon. The unused p orbitals in this in
this hybridization join side-to-side overlap to form a bond
called a pi bond. The pi bonds prevents rotation in the
system. The alkene is a planar structure with a C=C bond
length of 1.34 degree and a C-H bond length of 1.1 degree.
All bond angles are 120 degrees.
 NOMENCLATURE:
 Alkenes are named from the corresponding alkane
name by dropping the ane and by adding ene. The alkene
function is a higher priority than alkyl group and thus it
is given the lowest number in the chain.
 Alkene IUPAC Name Common Name
 CH2=CH2 ethane ethylene
 CH2=CHCH3 propene propylene
 CH2=CHCH2CH3 1-butene butylene
 The numbering of the position of the alkene is very
important, the numbering always goes across the double
bond.
 Ex. CH2=CHCH2CH3 1- butene
 CH3CH=CHCH3 2- butane
 Alkenes with alkyl substituents are named by
establishing the parent name of the longest chain alkene
and assigning the lowest number to the first double bonded
carbon of the alkenes. The alkyl substituents are named in
alphabetical order along with their position number. Ex.
 CYCLIC ALKENES:
 Cyclic alkenes are numbered from the alkenes (and
across the alkene) with the first carbon of the alkene
assigned number 1. The number 1 is written only when
required to show the position of a substituent, not to show
the position of the double bond that is assigned to position
1.
 Ex.
 NATURALLY OCCURING ALKENES:
 Alkene stuctures are also found in lemon oil, and turpentine, a paint
thinner is found in pine tress.
 The compound shaped like a soccer ball is not found in nature but is
obtained by passing an electrical arc through pure carbon electrodes. It
is named after Buckminster Fuller who proposed such structures.Thus
the name is fullerene, but the compound is sometimes referred to as a
Bucky Ball. It has the formula C60 and thus contain no hydrogen atoms.
 Low molecular weight alkenes are obtained by “cracking” or breaking
down alkanes at high temperature with special catalysts. Ethene is
obtained from ethane and some natural gas wells contain ethane, which
is an important resource for the production of polyethylene used in
plastic bags and toys.
 PHYSICAL PROPERTIES OF ALKENES:
 Alkenes like alkanes, are non-polar materials with generally
lower boiling points than the corresponding alkane. Alkenes
also have a strong odors, sometimes pleasant, sometimes not.
 PREPARATION OF ALKENES:
 Alkenes are often prepared by removal of substituents from
saturated precursors. Dehydration (loss of water) and
dehydrohalogenation (loss of H and halogen) are two common
elimination procedures for the preparation of alkenes
 Ex.
 REACTIONS OF ALKENES:
 Hydrogenation and Alkene Stability;
 A very important reaction of alkenes is the addition of
hydrogen to the double bond. A catalyst is required and the
reaction is called catalytic hydrogenation. Catalytic
hydrogenation provides alkanes in excellent yields and is an
important and convenient reaction alkanes is reacted under
pressure with hydrogen in the presence of a metal catalyst
ALKYNES…………..
 STRUCTURE:
 Alkynes, also called acetylenes, contain a structural unit in
which carbon is bonded to another carbon with three bonds. The
double unsaturation gives rise to the general formula of CnH2n-2
for alkynes. The carbon atoms in an alkyne are hybridized. The
single bonds are made of sp hybridized carbon atoms overlapped
with hydrogen or carbon. The remaining two unhybridized p-
orbitals join in side-to side overlap to make two orthogonal p-
bonds. The overall alkyne structure is linear with a C-C bond
length of 1.20 A and C-H bond length of 1.08 A.
 NOMENCLATURE:
 The IUPAC system of nomenclature derives an alkyne
name from corresponding alkane. The –ane is replaced by –
yne and a number is used to give the location of the triple
bond. Ethyne, CnH2, is the first alkyne and is also commonly
known as acetylene. Alkynes are named by locating the
parent name from the longest chain containing the triple
bond. The Alkyne is given the lowest possible number, and
numbering proceeds across the triple bond. Substituents are
given the lowest possible and are named alphabetically.
 NATURAL ALKYNES:
 Many alkynes are found in small amounts in plants
sources. The marigold flowers contains the bithienyl
compound. The plant uses this material to kill small worms
that attack the roots of the plant. Some gardeners grow
marigolds along with other plants to take advantage of this
natural protection.
 PREPARATION OF ALKYNES:
 Most alkynes are synthesized from acetylene. Pure
acetylene is obtained from calcium carbide, a substance
produced from carbon and calcium oxide.
 Acetylene can also be converted to sodium salt, sodium
acetaldehyde, that will displaced the halogen atom in
certain haloalkanes.
 PHYSICAL PROPERTEIS OF ALKYNES:
 Alkynes are low boiling materials that posses a high heat
content. Thus a major use for alkynes, especially acetylene,
is a fuel for high temperature flames. Acetylene is fairly
unstable and denotes under pressure. Pure acetylene is not
stored as a liquid or as a gas under pressure. It is absorbed
onto carbon and kept in steel containers at about 10 atm of
pressure. The acetylene flame can burn at a temperature
up to 2600. The boiling points of akynes are somewhat
higher than the boiling point of alkenes.
 REACTIONS OF ALKYNES:
 Addition of hydrogen to alkynes can be controlled to add
two hydrogen atoms and give an alkenes as a product, or
four hydrogen atoms to give an alkanes. Complete
reduction of acetylenes to hydrocarbons occurs easily on
hydrogenation with a number of catalyst including
Palladium on carbon.
 Alkynes react with a number of reagents to give addition
products in a manner similar to alkenes.
 SUMMARY:
 Hydrocarbons are a class of organic compounds only
hydrogen and carbon. Hydrocarbons are divided into
classes called alkanes, akenes and alkynes.
 Alkanes, CnH2n+2 are sp3 hybridized and have a
tetrahedral shape. Usually volatile and found abundantly
in oil and natural gas, alkanes are used widely as fuels.
The combustion of alkanes, reaction with oxygen,
produces carbon dioxide and water. Alkanes are
systematically named, IUPAC rules, by adding an ane suffix
to the term that designates the number of carbon atoms.
 ALKENES
 Alkenes, CnH2n, are sp2 hybridized and have a planar
shape. Alkenes are prepared by dehydration and
dehydrohalogenation reactions. Alkene chemistry is
dominated by reactions of the double bond called pi bond.
The double bond undergoes reduction, oxidation and
addition reactions. Alkenes are named by replacing the
alkane suffix, ane, with a new suffix ene. A number is
assigned to the position of the double bond.
 ALKYNES
 Alkynes, CnH2n-2, are sp hybridized and are linear.
Acetylene is well known for its use in welding torches.
The triple bond of alkynes can be reduced with hydrogen,
or oxidized by a variety of reagents. Alkynes are named
by replacing the ane suffix of the alkane with the yne
suffix. The triple bond location is designated by a
number.