Transcript Chapter 11

Chapter 11- Alkanes
Homework 11.12, 11.14, 11.15,
11.19, 11.22, 11.28, 11.29, 11.35,
11.43, 11.49, 11.51,11.53, 11.67
Alkanes
• Alkanes are the simplest type of organic
compound
• They are members of a larger class called
hydrocarbons
• Hydrocarbons- a compound composed of
only carbon and hydrogen
Hydrocarbons
Alkanes
Alkenes
Alkynes
Arenes
• Alkanes are saturated hydrocarbons,
meaning they only contain carbon-carbon
single bonds.
• A hydrocarbon that contain carbon-carbon
double bonds, triple bonds or benzene rings
is said to be unsaturated.
• Structure of Alkanes:
• Each carbon has four bonds with bond
angles of 109.5o, and is tetrahedral in shape.
• As we add carbons, the structure gets more
complex, but each carbon is still tetrahedral.
Naming Alkanes
Group Nomenclature
Short hand
• We can condense the condensed formula
further by putting like groups together in
parenthesis with a subscript.
Constitutional Isomers
• Constitutional Isomers- compounds that
have the same molecular formula but different
structural formulas.
• By different structural formulas, we mean that
they can either have different types of bonds
and/or differ in their connectivity.
• For CH4, C2H6, and C3H8, there are no
constitutional isomers
– No variations!!
• For C4H10, there are two:
• These are different compounds and have
different physical and chemical properties!!
• All compounds that have the same molecular
formula but different structural formulas are
constitutional isomers.
Examples
• 2 alcohols C3H8O
• 2 1o amines C3H9N
• 2 aldehydes C4H8O
• 2 carboxylic acids C4H8O2
• The ability of carbon atoms to form strong,
stable bonds with other carbon atoms results
in a staggering number of constitutional
isomers.
CH41
C5H12- 3
C10H22- 75
C15H32- 4,347
C25H52- 36,797,588
C30H62- 4,111,846,763
Nomenclature of Alkanes
• Ideally, every organic compound should
have a name from which its structural
formula can be drawn.
• The rules are established by the
International Union of Pure and Applied
Chemistry.
• They are called IUPAC names for short.
Unbranched Alkanes
• IUPAC names for unbranched chains of
carbon consist of two parts:
– 1) Prefix- shows the number of carbons in the
chain
– 2) Suffix- -ane, to show that the compound is
an alkane
Prefixes-
Branched Alkanes
• The IUPAC name for alkanes with
branched chains consist of a parent name,
which shows the longest chain of carbons,
and substituent names that indicate the
groups attached to the parent chain.
• Alkyl Group- a substituent group derived
from an alkane. It is named by dropping the
–ane and adding –yl.
Examples of Alkyl groups
Rules of the IUPAC system
1)The name for an unbranched chain consists of a
prefix showing the number of carbons and the
ending –ane.
2) For branched chains, find the longest continuous
chain as the parent chain, and its name is the root
name.
3) Give each substituent on the parent chain a name
and number which corresponds to the carbon
atom the substituent is bonded to. Use a hyphen
to separate numbers and words.
4) If there is only one substituent, number the
parent chain from the end that gives the
substituent the lowest number.
example:
5) If the same substituent occurs more than
once, number the parent chain to give the
lowest set of numbers and use di, tri, tetra,
penta, hexa, etc. to indicate the number of
times the substituent appears. Separate
numbers with commas.
Examples
6) If there are more than one type of
substituent, list them alphabetically and
number the parent chain to obtain the lowest
set of numbers. If there are different
substituents in equal positions, give the one
that come first alphabetically the lowest
number.
examples:
7) Do not include the numerical prefixes di,
tri, tetra, etc. in alphabetizing. You also do
not include sec- or tert- in alphabetizing.
examples.
SKIP
• We are skipping section B, common names,
on page 330.
Sources of Alkanes
• Major sources: Natural Gas and Petroleum
• Natural gas consists of about 90-95% methane, 510% ethane, and some propane, butane, and 2methylpropane
• Petroleum is a thick, viscous liquid mixture of
1000’s of compounds, most of them
hydrocarbons, formed from the decomposition of
marine plants and animals
Sources of Alkanes
• The fundamental separation process in
refining petroleum is fractional distillation
• Practically all crude petroleum that enters a
refinery goes to distillation units, where it is
heated to temperatures as high as 370 to
425oC and separated into fractions
• Each fraction contains a mixture of
hydrocarbons that boil within a particular
range.
Cycloalkanes
• A hydrocarbon that contains carbon atoms
joined to form a ring are called cyclic
hydrocarbons.
• When all carbons of the ring are saturated,
the hydrocarbon is called a cycloalkane.
– Examples:
Naming
• To name a cycloalkane, use the prefix cyclo
and the parent name that corresponds to the
number of carbons in the ring.
• Then name each substituent on the ring, if
there are any.
• If there is only one substitituent, there is no
need to give the locate or position number.
• If there are two substituents, number the
ring beginning with the substituent that
comes first alphabetically.
– Examples:
Shapes of Alkanes and Cycloalkanes
• Organic Molecules are 3-D
• Alkanes
– Free rotation about each C-C bond
• Conformation- Any 3-D arrangement of
atoms in a molecule that results from rotation
about a single bond
• There are an infinite number of conformations
due to this rotation
• Molecules will spend most of there time in the
lowest energy conformations
Shapes of Alkanes and Cycloalkanes
• Cycloalkanes
– Rotation is restricted due to ring structure
• Cyclopentane- most staple conformation is
the envelope conformation
– 4 carbons in a plane and 1 carbon that is bent
out of the plane.
– All bond angles are 109.5o
Shapes of Alkanes and Cycloalkanes
• Cyclohexane- most stable in the chair
conformation, all bond angles 109.5o
• The 12 C-H bonds are arranged in two
different orientations
• 6 are Axial bonds, the other 6 are
Equatorial bonds.
• Axial bonds extend straight up or down
• Equatorial bonds extend out from the
parameter of the ring
Cis/Trans Isomerism
• Cycloalkanes with substituents on two or
more carbons of the ring show a type of
isomerism
• Cis/trans isomers have:
– Same Molecular formula
– Same order of attachment
– Different arrangement of their atoms in space
due to the restricted rotation around the C-C
single bonds of the ring
Cis/Trans Isomerism
• The prefix cis- indicates that the substituents
are on the same side of the ring
• The prefix trans- indicates that the substituents
are on the opposite side of the ring.
• Cis/trans isomers differ in the orientation of
their atoms in space, so they are called
stereoisomers
• Cis/trans are only one form of stereoisomers
Physical Properties of Alkanes
• Most important is the almost complete lack of
polarity in cycloalkanes and alkanes
• Melting/Boiling Points
– In general, both increase as you increase MW
• 1-4 carbons………gases at RT
• 5-17 carbons…….liquids at RT
• 18+ carbons……..white, waxy solids
• As branching increases, the surface area
decreases, so London Forces are weaker,
which lead to lower MP/BP
Physical Properties of Alkanes
• Solubility
– Alkanes are not soluble in water
– They are soluble in each other
• Reactions of Alkanes
– The most important chemical property of
alkanes is their inertness
– They are quite unreactive toward any of the
normal ionic reaction conditions.
Reactions of Alkanes
1) Combustion
– Oxidation of hydrocarbons is the basis for their uses as
energy sources for heat and power
CH4 + 2O2
CH3CH2CH3+5O2
CO2 + 2H2O
+212 kcal/mol
3CO2+4H2O +530 kcal/mol
Reactions of Alkanes
2) Reactions with Halogens: Halogenations
CH4 + Cl2
heat or light
CH3Cl + HCl
In IUPAC naming, the name for haloalkanes
comes from naming the halogen atom as a
substituent and alphabetizing it along with
other substituents.
F= Fluoro, Cl= Chloro, Br = Bromo, I= Iodo