Transcript Document

Chemistry 20
Chapter 2
Alkanes
Review
Carbon
• Carbon has four valence electrons; hydrogen has one.
•
• C •
H•
•
• To obtain an octet, carbon forms four bonds.
H
••
HC H
••
H
H
H
H
C
H
H
CH4 , methane
Hydrocarbons
Large family of organic compounds
Composed of only carbon and hydrogen
Saturated hydrocarbons
Unsaturated hydrocarbons
Alkanes
H
Alkenes, Alkynes
& Aromatics
H
C-C
C=C
H
C
C
C
C
C
H
C C
C
H
Alkanes
Methane
Tetrahedral
Expanded structural formula (Lewis):
showing each bond line.
Molecular formula
CH4
Ethane
C2H6
Molecular formula
Expanded structural formula
CH3 – CH3
Condensed structural formula: with each carbon atom and
its attached hydrogen atoms.
Alkanes
CnH2n+2
n: number of carbon atoms
Naming of Alkanes
Prefix + ane
CnH2n+2
Line-angle Formula
Propane
CH3-CH2-CH3
Butane
CH3-CH2-CH2-CH3
Pentane
CH3-CH2-CH2-CH2-CH3
Naming Substituents
In the IUPAC system:
• Removing a H from an
alkane is called alkyl
group.
-ane
-yl
• Halogen atoms are
named as halo.
-ine
-O
-OH
-NO2
Hydroxyl
Nitro
STEP 2
Number the carbon atoms starting
from the end nearest a substituent.
STEP 3
Give the location and name of each
substituent (alphabetical order) as a
prefix to the name of the main chain.
Give the name of:
CH3

CH3─CH─CH2─CH3
STEP 1
Longest chain is butane.
STEP 2
Number chain.
CH3

CH3─CH─CH2─CH3
1
2
3
4
STEP 3 Locate substituents and name.
2-Methylbutane
Give the name of:
CH3 CH3


CH3─CH─CH─CH3
STEP 1
Longest chain is butane.
STEP 2
Number chain.
CH3 CH3


CH3─CH─CH─CH3
1
2
3
4
STEP 3 Locate substituents and name.
2,3-dimethylbutane
Cl CH3


CH3─CH2─CH─CH─CH3
STEP 1
Longest chain is pentane.
STEP 2
Number chain from end nearest substituent.
Cl CH3


CH3─CH2─CH─CH─CH3
5
4
3
2
1
STEP 3 Locate substituents and name alphabetically.
3-Chloro-2-methylpentane
CH3
CH3
|
|
CH3─CH─CH2 ─CH─CH3
1
2
3
4
2,4-dimethylpentane
5
Cl
CH3
|
|
CH3─CH2─CH─CH2─C─CH2─CH3
|
Cl
7
6
5
4
3
2
1
3,5-dichloro-3-methylheptane
CH2 CH3

CH3─CH─CH2─CH3
STEP 1
Longest chain has 5 carbon atoms (Pentane).
STEP 2
Number chain from end nearest substituent.
1
2
CH2 CH3

CH3─CH─CH2─CH3
3
STEP 3
4
5
Locate substituent and name.
3-Methylpentane
Constitutional Isomers
• Have the same molecular formula.
• Have different atom arrangements (different structural formula).
CH3CH2CH2CH3
Butane
C4H10
2-Methylpropane
C4H10
CH3
CH3CHCH3
Cyclic Hydrocarbon - Cycloalkane
Carbon atoms are joined to form a ring.
=
Cyclobutane
=
Cyclopentane
=
Cyclohexane
Naming of Cycloalkanes
Prefix “cyclo-” + the name of the open-chain alkane
Substituents:
- One substituent: no location number
- Two or more substituents: number the ring beginning
with the substituent of lower alphabetical order.
OH
Cl
2
CH3
1
Chlorocyclohexane
Br
1-Bromo-2-methylcyclohexane
3
1
Cl
2
NO2
1-Chloro-3-hydroxyl-2-nitrocyclopentane
Conformation - 3D shapes
Any three-dimensional arrangement of atoms in a molecule that results by rotation
about a single bond.
– The following are three conformations for a butane molecule.
1
2
rotate
by 120°
3
rotate
by 60°
4
Least crow ded
conformation
Intermed iate
crow din g
Most crow d ed
conformation
Conformation - 3D shapes
• The most stable conformation of a cyclopentane is the envelope
conformation.
• The most stable conformation of a cyclohexane is the chair conformation.
– All bond angles are approximately 109.5°.
Conformation - 3D shapes
•
In a chair conformation,
– six C-H bonds are equatorial bonds.
– six C-H bonds are axial bonds.
axis th rough the
cen ter of th e rin g
H
H
H
H H
H
H
H
(a) Ball-and-s tick mod el
sh ow ing all 12 hydrogen s
H
(b) The s ix eq uatorial
C-H
bond s
Equatorial
H
H
H
(c) The s ix axial
C-H bAxial
on ds
Conformation - 3D shapes
– The more stable conformation of a substituted cyclohexane ring has the
substituent group(s) as equatorial rather than axial.
H H
H
H
CH3
CH3
Equatorial methylcyclohexane
More Stable
Axial methylcyclohexane
Cis & Trans Stereoisomers
The same molecular formula and the same connectivity of their atoms,
but a different arrangement of their atoms in space.
H
H
H
H H
H
H
H
H
H
H H H3 C
H
CH 3
CH3
cis-1,2-D imeth ylcyclop entane
H
H
CH3
H
trans -1,2-D imethylcyclop entane
mp & bp of cis < mp & bp of trans
Cis & Trans Stereoisomers
CH3
H
CH3
H
CH3
H
or
H3 C
H
CH3
trans -1,4-Dimethylcyclohexane
or
H3 C
CH3
CH3
cis-1,4-Dimethylcyclohexane
Stereoisomers: Isomers that have the same molecular formulas and the same
connectivity of their atoms but a different orientation of their atoms in space.
Physical Properties of Alkanes
•
•
•
•
•
Nonpolar
Insoluble in water.
Lower density than water.
Low boiling and melting points.
Gases with 1-4 carbon atoms.
(methane, propane, butane)
• Liquids with 5-17 carbon atoms.
(kerosene, diesel, and jet fuels)
• Solids with 18 or more carbon atoms.
(wax, paraffin, Vaseline)
Boiling & melting points of Alkanes
Number of carbon atoms ↑
Number of branches ↑
CH3CH2CH2CH3
bp & mp ↑
bp & mp ↓
CH3
CH3CHCH3
Chemical reactions of Alkanes
Low reactivity
1- Combustion:
• Alkanes react with oxygen.
• CO2, H2O, and energy are produced.
• Alkane + O2
CH4 + 2O2
CO2 + H2O + heat
CO2 + 2H2O + energy
Chemical reactions of Alkanes
Low reactivity
2- Halogenation:
Alkanes react with Halogens.
CH4 + Cl2
CH3Cl + HCl
Chloromethane
CH2Cl2 + HCl
Dichloromethane
CHCl3 + HCl
Trichloromethane
CCl4 + HCl
Tetrachloromethane
Heat or light
CH3Cl+ Cl2
CH2Cl2 + Cl2
CHCl3 + Cl2
Heat or light
Heat or light
Heat or light
Sources of Alkanes
• Natural gas
– 90 to 95 percent methane.
– 5 to 10 percent ethane, and
– a mixture of other low-boiling alkanes, chiefly propane, butane, and 2methylpropane.
• Petroleum
– A thick liquid mixture of thousands of compounds, most of them
hydrocarbons, formed from the decomposition of marine plants and
animals.