Chapter 2
Alkanes
ORGANIC CHEMISTRY
▪ Definition – the study of carbon-containing
compounds and their properties
▪ There are a few inorganic compounds that contain
carbon: carbon and its oxides and carbonates.
▪ Most compounds containing carbon are organic
compounds.
▪ These typically contain chains or rings of carbon
atoms.
HYDROCARBONS
▪ The simplest class of organic compounds.
▪ Hydrocarbons are compounds formed from carbon
and hydrogen.
▪ Saturated hydrocarbons are those that contain all
single carbon-carbon bonds.
▪ Unsaturated hydrocarbons contain either double or
triple carbon-carbon bonds.
ALKANES
▪ Hydrocarbons with only single C-C bonds.
▪ The carbons are surrounded by 4 atoms and
assume the tetrahedral
shape and have bond
。
angles of 109.5
▪ Normal alkanes are formed from strings of
carbon atoms attached to each other.
▪ Due to the tetrahedral bond angles, they are
not really straight chains, but zigzag shaped.
ALKANES (con’t)
▪ The normal alkanes are also called unbranched
hydrocarbons.
▪ The general formula is CnH2n+2.
▪ Alkanes are saturated with hydrogen (no more can
be added).
▪ They are also called aliphatic compounds
▪ As the carbon chains get longer, molar mass
increases and melting and boiling points increase.
▪ Methane, CH4 is the simplest hydrocarbon..
Tetrahedral Structure of Carbon
VSEPR theory predicts that a
carbon atom with four single
covalent bonds is tetrahedral
as shown in a
a.
b.
c.
d.
tetrahedron.
ball-and-stick model.
space-filling model.
expanded structural
formula.
Tetrahedral shape of Ethane, C2H6
In ethane,
▪ each carbon atom is bonded to another carbon atom and three
hydrogen atoms.
▪ each carbon atom has a tetrahedral shape.
Representations of ethane, C2H6: (a) tetrahedral shape of each carbon, (b) balland-stick model, (c) space-filling model, (d) expanded structural formula.
Names of Alkanes
The names of alkanes
▪ are determined by the IUPAC (International Union of
Pure and Applied Chemistry) system.
▪ use stem names to indicate the number of carbons in
a chain.
▪ end in –ane.
ORGANIC STEM NAMES
Stems That Indicate the Number of Carbon Atoms in Organic
Molecules
Stem
methethpropbutpenthexheptoctnondec-
Number
1
2
3
4
5
6
7
8
9
10
Names of Parent Alkanes
Structural Formulas
Alkanes are written with structural formulas that are
▪ expanded to show each bond.
▪ condensed to show each carbon atom and the
attached hydrogen atoms.
Line-Bond Formulas
▪ Because each C atom has a tetrahedral
arrangement, the order of atoms is not a straight
line, but a zigzag pattern.
▪ A line-bond or skeletal formula abbreviates the
carbon atoms and shows only the zigzag pattern of
bonds between carbon atoms.
Heptane has 7 carbon atoms.
Corners represent carbon atoms, H atoms are not drawn
Types of Structural Formulas
Butane, C4H10 Structural Representations
Drawing Structural Formulas
A molecule of pentane, C5H12 has five carbon atoms
connected to each other and to hydrogen atoms with
single bonds to give each carbon four bonds.
In a condensed structural formula, carbon atoms
on each end are written as CH3—.
in the middle are written as —CH2—.
The skeletal formula is a zigzag line where the ends
and corners represent carbon atoms.
Drawing Structural Formulas
1. Give the name of each of the following compounds.
A. CH3—CH3
ethane
B. CH3—CH2—CH3
propane
C. CH3—CH2—CH2—CH2—CH2—CH3
hexane
2. Write the condensed structural formula of pentane.
CH3—CH2—CH2—CH2—CH3
1. Draw the condensed structural formula of
H
H
H
H
H
H
H
C
C
C
C
C
C
H
H
H
H
H
H
H3C
CH2
CH2
CH2
CH2
CH3
H
A. What is its molecular formula?
C6H14
B. What is its name?
hexane
Draw the condensed structural formula for the
following alkanes.
A. ethane
CH3─CH3
B. heptane
CH3─CH2─CH2─CH2─CH2─CH2─CH3
Stereochemical Formulas
Show the relative positions of atoms and groups around
carbon in three dimensions.
▪ Two bonds are drawn in the plane of the page
▪ A bond sticking out of the front of the page is
represented by a solid triangular wedge.
▪ A bond sticking out of the back of the page is
dotted line (dashed).
Goes back
Comes forward
in the plane of
the paper
Stereochemical Formulas
Cyclic Compounds:
▪ Two bonds drawn in the plane of the page (sticks),
one bond above the ring (wedged), and one bond
below the ring (dashed).
▪ Groups with the same orientation (both wedged or
both dashed) are cis to one another, and groups with
opposite orientations (one wedged and one dashed)
are trans to one another.
Substituents
Alkanes with four or more carbons can have side groups
called branches or substituents,
▪ which are attached to the carbon chain.
▪ that replace at least one H on the alkane chain.
▪ Example: alkyl group or halo group
Alkyl Groups
▪ are carbon branches.
▪ remove one H from an alkane (a part of a structure)
▪ General abbreviation “R”
▪ are named by replacing the -ane ending of the
▪ corresponding alkane name with yl.
methane = methyl
—CH3
ethane = ethyl
—CH2—CH3
propane = propyl
—CH2—CH2—CH3
Some Straight-Chain Alkyl Groups
Common Alkyl Groups
Primary, Secondary and Tertiary Carbon
Primary carbons are connected to one carbon only.
Secondary carbons are connected to two carbon atoms.
Tertiary carbons are connected to three carbon atoms.
And if four carbons are connected to a carbon, then it is a quaternary carbon.
Naming Alkanes with Substituents
Naming Alkanes with Substituents
Name the following alkane.
CH3
CH
CH2
CH2
CH3
Step 1 Write the alkane name
of the longest chain
of carbon atoms.
pentane
Step 2 Number the carbon
atoms starting from
the end nearer a
substituent.
CH3
CH3
CH
CH2
CH3
1
2
CH3
CH
CH3
CH2
CH3
3
CH2
4
CH2
CH3
5
Naming Alkanes with Substituents
Step 3 Give the location and name of each
substituent (alphabetical order) as a prefix
to the name of the main chain.
2-methylpentane
1
2
CH3
CH
CH3
3
CH2
4
CH2
CH3
5
The position of the substituent group is given by a number
followed by a hyphen in front of its name showing the carbon
atom to which it is attached.
Naming Alkanes with Substituents
▪ If there are more than one substituent group of
the same kind, use the prefixes di-, tri-, tetrabefore the name.
▪ If there is more than one type of substituent
group, they are put in alphabetical order.
Naming Alkanes with Alkyl Groups
methylpropane
2,4-dimethylpentane
What is the alkane name
of the following?
Step 1 Write the alkane name of the longest chain of
carbon atoms.
4 C's = butane
Step 2 Number the carbon atoms starting from the
end nearer a substituent.
Solution
Step 3 Give the location and name of each
substituent (alphabetical order) as a prefix
to the name of the main chain.
2-methylbutane
Examples
4-ethyl-2-methylhexane
2,6,6-trimethyloctane
3,3,4-trimethylhexane
Give the structures of the followings:
a) 4-isopropyloctane and
b) 5-t-butyldecane.
Solution: 4-Isopropyloctane has a chain of eight carbons, with an isopropyl
group on the fourth carbon. 5-t-Butyldecane has a chain of ten carbons,
with a t-butyl group on the fifth.
Naming Haloalkanes
In a haloalkane,
▪ halogen atoms replace hydrogen atoms.
▪ are named fluoro (—F), chloro (—Cl), bromo (—Br) or
▪
iodo (—I).
▪ substituents are numbered and arranged alphabetically.
▪ alkanes with halogens can be named as alkyl halides;
the carbon group is named as an alkyl group followed
by the halide name.
CH3—CH2—Cl
CH3—Br
IUPAC Name
chloroethane
bromomethane
Common Name ethyl chloride
methyl bromide
What is the IUPAC name for the following?
Step 1 Write the alkane name of the longest chain
of carbon atoms.
5 C's = pentane
Step 2 Number the carbon
atoms starting from
the end nearer a
substituent.
Solution
Step 3 Give the location and name of each
substituent (alphabetical order) as a prefix
to the name of the main chain.
3-chloro-2-methylpentane
Give the IUPAC name for each of the following.
2,4-dimethylhexane
1
2
3
4
5
6
more substituents on C3
7
6
5
4
3
2
1
3,5-dichloro-3-methylheptane
Examples of Naming Haloalkanes.
Guide to Drawing Alkane Structures
Draw the condensed structural formula for
3-bromo-1-chlorobutane.
Step 1 Draw the main chain of carbon atoms.
C─C─C─C
Step 2 Number the chain and place the substituents
on the carbons indicated by the numbers.
Step 3 Add the correct number of hydrogen atoms
to give four bonds to each C atom.
Cycloalkanes
▪ General Formula: CnH2n
▪ are carbon rings.
▪ have two fewer hydrogen atoms than straight-chain
alkanes with the same number of carbon atoms.
▪ are named by using the prefix cyclo- before the
name of the alkane chain.
Cycloalkanes: CnH2n
Name the following alkanes.
butane
A. CH3─CH2─CH2─CH3
B.
cyclobutane
C. CH3─CH2─CH2─CH2─CH2─CH2─CH2─CH3
D.
cyclohexane
octane
Naming Cycloalkanes with Substituents
When naming a cycloalkane with a substituent
▪ the name of the substituent is placed in front of
the cycloalkane name.
▪ the number of the substituent is not needed when
there is only one substituent.
If two or more substituent’s are attached, the ring is
numbered by assigning carbon 1 to the substituent that
comes first alphabetically.
Naming Cycloalkanes with Substituents
The name of a substituent is placed in front of the
cycloalkane name.
Methylcyclobutane
When there are two substituents, the ring is
numbered by assigning carbon 1 to the substituent
that comes first alphabetically.
1-bromo-2-chlorocyclopentane.
Geometric Isomers
1
CH3
2
CH3
2
Same side: ciscis-1,2-dimethylcyclohexane
CH3
Opposite side: trans1
CH2CH3
trans-1-ethyl-2-methylcyclohexane
Learning Check
Name each of the following:
methylcyclopropane
1-chloro-3-ethylcyclohexane
Chlorination of Methane
▪ Requires heat or light for initiation.
▪ The most effective wavelength is blue, which is
absorbed by chlorine gas.
▪ Many molecules of product are formed from
absorption of only one photon of light (chain
reaction).
Physical Properties
Alkanes are
▪ nonpolar.
▪ insoluble in water.
▪ less dense than water (1.0 g/mL) with a density from
(0.62 g/mL to 0.79 g/mL).
▪ Boiling point and melting point depends on the
molecular weight.
Alkanes: Melting and Boiling Points
Alkanes
▪ have the lowest melting and boiling points of all
organic compounds.
▪ have only nonpolar bonds, C—C and C—H.
▪ The longer the straight chain, the higher boiling
point.
Branched Alkanes: Melting and
Boiling Points
The boiling and melting points of branched alkanes are
lower than their straight-chain isomers because they are
more compact.
There is more contact between
hexane molecules (red and black
licorice) than between the surfaces
of 2,3-dimethylbutane molecules
(tennis balls).
For the following pairs of hydrocarbons, which one
has the higher boiling point? Why?
1. Butane or octane
Octane (has more carbon atoms) and, therefore, a
higher boiling point.
2. Hexane or 2,3-dimethylbutane
Both alkanes have the same number of carbon
atoms. Hexane is not branched and, therefore, has
the higher boiling point.
Conformations of Ethane
▪ Rotation is possible around C-C bonds in open-chain
molecules.
▪ The different arrangements of atoms that result from
bond rotation are called conformations.
▪ Molecules that have different arrangements are
called conformational isomers, or conformers.
▪ Different conformers can’t be isolated because they
interconvert too rapidly.
Conformations of Ethane
Conformations can be represented in 2 ways:
▪
Sawhorse representations show
molecules at an angle, showing a
molecular model
▪ C-C bonds are at an angle to
the edge of the page and all
C-H bonds are shown
▪
Newman projections show how
the C-C bond would project endon onto the paper
▪ Bonds to front carbon are
lines going to the center
▪ Bonds to rear carbon are lines
going to the edge of the circle
Conformations of Ethane
▪ Staggered- most stable: all 6 C-H bonds are as far
away as possible
▪ Eclipsed- least stable: all 6 C-H bonds are as close
as possible to each other
▪ Torsional strain - arises when all the bonds on
neighboring atoms are eclipsed.
Conformations of Propane
Stabilities of Some Cycloalkanes
▪ Carbons of cycloalkanes are sp3 hybridized and thus
require an angle of 109.5º.
▪ When a cycloalkane has an angle other than 109.5º,
the compound will have angle strain.
▪ Angle strain - expansion or compression of bond
angles away from most stable
Cyclopropane: C3H6
▪ The bond angles are compressed to 60° from the usual
109.5° bond angle of sp3 hybridized carbon atoms.
▪ This severe angle strain leads to nonlinear overlap of the
sp3 orbitals and “bent bonds”.
▪ All the C—C bonds are eclipsed, generating torsional
strain that contributes to the total ring strain.
Conformations of Cyclobutane
▪ Cyclobutane has less angle strain than cyclopropane but
more torsional strain because of its larger number of ring
hydrogens
▪ Cyclobutane is slightly bent out of plane - one carbon
atom is about 25°above
▪ The bend increases angle strain but decreases
torsional strain
Conformations of Cyclopentane
▪ Planar cyclopentane would have no angle strain but
very high torsional strain
▪ Actual conformations of cyclopentane are nonplanar
(slightly puckered rather than flat), reducing torsional
strain
▪ Four carbon atoms are in a plane
▪ The fifth carbon atom is above or below the plane –
looks like an envelope
Chair Conformations of Cyclohexane
▪ Cyclohexane eliminates eclipsing and angle strain by
adopting non-planar conformations.
▪ The most stable conformation of cyclohexane is called
the “chair“ conformation.
▪ It has no torsional strain as all adjacent C-H bonds
are staggered.
▪ The bond angles are close to 109°.
How to Draw Cyclohexane
Axial and Equatorial Bonds in
Cyclohexane
▪ The chair conformation has
two kinds of positions for
substituents on the ring: axial
positions and equatorial
positions
▪ Chair cyclohexane has six
axial hydrogens
perpendicular to the ring
(parallel to the ring axis) and
six equatorial hydrogens
near the plane of the ring
Drawing the Axial and Equatorial
Hydrogens
66
Conformational Mobility of
Cyclohexane
▪ Chair conformations readily interconvert, resulting in the
exchange of axial and equatorial positions by a ring-flip
Monosubstituted cyclohexanes
▪ A monosubstituted cyclohexane is one in which there is one
non-hydrogen substituent in the cyclohexane ring.
▪ The most energetically favorable conformation for a
monosubstituted cyclohexane is the chair conformation with the
non-hydrogen substituent in the equatorial position because it
prevents high steric strain from 1,3 diaxial interactions.
▪ Steric strain: repulsive interactions between nonbonded atoms
in close proximity
The conformer of methylcyclohexane with equatorial methyl is
favored by 1.74 kcal/mol (7.3 kJ/mol) relative to the conformer where
methyl is axial.
Functional Groups
▪ Functional group collection of atoms at a site
that have a characteristic
behavior in all molecules
where it occurs
▪ The group reacts in a typical
way, generally independent
of the rest of the molecule
▪ For example, the double
bonds in simple and
complex alkenes react with
bromine in the same way
Functional Groups with
Multiple Carbon–Carbon Bonds
▪ Alkenes have a C-C double bond
▪ Alkynes have a C-C triple bond
▪ Arenes (aromatic hydrocarbons) have special
bonds that are represented as alternating single
and double C-C bonds in a six-membered ring.
These compounds are hydrocarbons that
contain a benzene ring
Functional Groups with Carbon Singly
Bonded to an Electronegative Atom
Groups with a Carbon–Oxygen Double Bond
(Carbonyl Groups)
Structures of Some Functional Groups
Structures of Some Functional Groups
Structures of Some Functional Groups