Hydrocarbons
Alkanes, Cycloalkanes, Alkenes Alkynes and Aromatics
Hydrocarbons
Hydrocarbons - compounds containing only hydrogen and carbon
obtained by physical separation from petroleum and natural gas from oil and gas refining Refining - separates complex mixtures into purified components
Petroleum - is a mixture of hundreds of thousands of compounds having boiling points range from 20oC to more than
400oC the difference in boiling points allows the separation of these
compounds by fractionation or fractional distillation
Fractional distillation tower on page 296
Reactions of Hydrocarbons
Cracking – larger hydrocarbons are broken into smaller fragments in the absence of air
Example: C17H36 (l) C9H20 (l) + C8H16 (l)
Reforming – smaller hydrocarbons combine to form larger ones; makes higher grade from lower grade gasoline
Example: C5H12 (l) + C5H12 (l) C10H22 (l) + H2 (g)
Combustion – hydrocarbons burn in the presence of oxygen to form carbon dioxide and water Example: C3H8 (g) + 5 O2 (l) 3 CO2 (l) + 4 H2O (g)
Alkanes
Alkanes – are hydrocarbons with only single C-C bonds – general formula: CnH2n+2
NAMING STRAIGHT CHAIN ALKANES Prefix+ ane prefix = indicates the # of C atoms IUPAC Name Formula methane CH4 (g) ethane C2H6 (g)
propane C3H8 (g) butane C4H10 (g)
pentane C5H12 (l) hexane C6H14 (l) heptane C7H16 (l) octane C8H18 (l) nonane C9H20 (l) decane C10H22 (l)
Naming Branched Chain Alkanes Branch - carbon atoms that are not part of the
parent (longest) chain of carbon atoms - name: prefixyl
Examples: Name Branch methyl -CH3 ethyl -C2H5 (or CH2-CH3) propyl -C3H7 (or CH2-CH2-CH3)
NAMING BRANCHED CHAIN ALKANES Step 1: Identify the longest continuous chain of carbon
atoms (the parent chain) and number the carbon atoms, starting from the end closest to the branch(es).
When there are two chains of equal length, choose the chain with the greater number of branches
When branching first occurs at an equal distance from
either end, number the carbons to give the lowest number at the first point of difference
Step 2: Identify any branches and their location number on the parent chain. Give each branch a number corresponding to its location on the longest chain.
Numbers are separated from words by a hyphen Branches should be listed in alphabetical order When two or more branches are identical:use the
prefixes di, tri, tetra, penta, etc. use commas to separate #s from each other
Step 3: Write the IUPAC name as follows: (# of location) - (branch name) (parent chain)
Parent chain = prefix + “ane” Branch name = prefix + “yl” # of location indicates C on parent chain that the
branch is attached to
*NOTE: use the lowest set of numbers to indicate branch
locations branches are listed in alphabetical order use “,” between #’s when there is more than one
branch use ”-“ to separate #’s from branch names use the prefix “di” “tri” or “tetra” + branch name for
two, three or four identical branches
Example #1: CH3-CH2-CH2-CH2-CH3
Step 1: CH3-CH2-CH2-CH2-CH3
Parent Chain: pentane
Step 2: CH3-CH2-CH2-CH2-CH3
Branch: none
Step 3: (# of location)-(branch name) (parent chain)
NAME: pentane
Example #2: CH3-CH-CH2-CH3
| CH3
Step 1: CH3- CH- CH2- CH3 | CH3
Parent Chain: butane
Step 2: CH3- CH- CH2- CH3 | CH3
Branch: 2-methyl
Step 3: (# of location)-(branch name) (parent chain)
NAME: 2-methylbutane
Example #3: CH3 | CH3- C- CH3
| CH3
Step 1: CH3 | CH3- C- CH3 l CH3
Parent Chain: propane
Step 2: CH3 | CH3- C- CH3 | CH3
Branches: 2,2-dimethyl
Step 3: (# of location)-(branch name) (parent chain) NAME: 2,2-dimethylpropane
Drawing Branched Chain Alkanes
Step 1: Draw a straight chain containing the number of atoms in the parent chain, and number the atoms from left to right.
Step 2: Attach all branches to their numbered locations on the parent chain.
Step 3: Add enough H atoms to show that each C has 4 bonds.
Example 1: Draw a condensed structural diagram for the following:
4-propylheptane
Step 1: C-C-C-C-C-C-C Parent chain: heptane
Step 2: C-C-C-C-C-C-C Branch: 4-propyl
|
C-C-C
Step 3: CH3-CH2-CH2-CH-CH2-CH2-CH3
|
CH2-CH2-CH3
Example 2: Draw a condensed structural diagram for the following:
2,4-dimethylpentane
Step 1: C-C-C-C-C Parent chain: pentane
Step 2: C-C-C-C-C Branches: 2,4-dimethyl
| |
C C
Step 3: CH3-CH-CH2-CH-CH3
| |
CH3 CH3
Example 3: Draw a condensed structural diagram for the following:
3-ethyl-2,4-dimethylhexane
Step 1: C-C-C-C-C-C Parent chain: hexane C-C | Step 2: C-C-C-C-C-C Branches: 3-ethyl & | | 2,4-dimethyl C C Step 3: CH2-CH3
| CH3-CH-CH-CH-CH2-CH3
| | CH3 CH3
Cycloalkanes
cycloalkanes: - are hydrocarbons that form a closed ring - all C-C bonds are single bonds - general formula: CnH2n
Naming: simply add cyclo in front of the alkane name
Examples: cyclopropane: CH2 / \ OR CH2-CH2
cyclobutane: CH2-CH2 | | OR CH2-CH2
cyclopentane: CH2
/ \ CH2 CH2 OR | | CH2 - CH2
Alkenes and Alkynes Alkenes - hydrocarbons with one or more
double carbon-carbon bonds (C=C) named with the suffix “ene” (table 9.5 page 303)
general form: CnH2n
example: CH2=CH2 name: ethene
Alkynes - hydrocarbons with one or more triple carbon-carbon bonds (C=C) named with the suffix “yne” (table 9.6 page 303)
general form: CnH2n-2
Example: CH=CH name: ethyne
Naming Alkenes and Alkynes
Named the same as alkanes with these additional considerations:
the longest chain must contain the multiple bond
the chain is numbered from the end closest to the multiple bond
the parent chain is preceded by a number indicating the position of the multiple bond
Examples: 1.) CH2=CH-CH2-CH3
name: 1-butene 2.) CH3-CH=CH-CH3
name: 2-butene 3.) CH3
| CH3-C=C-CH-CH3
name: 4-methyl-2-pentyne
Saturated versus Unsaturated
Hydrocarbons
Where have you heard the term “saturated” before? Saturated Hydrocarbons - are hydrocarbons that
contain only single bonds and therefore have the maximum number of hydrogen atoms possible (i.e. alkanes)
Example: CH3-CH2-CH3
Unsaturated Hydrocarbons - are hydrocarbons that contain at least one ring or multiple bond and have less than the maximum number of hydrogen atoms possible (i.e. cycloalkanes, alkenes and alkynes)
Example: CH3-CH=CH2
Reactions of Alkenes and Alkynes Unsaturated hydrocarbons like alkenes and alkynes can be
converted to alkanes through a chemical reaction called addition, or hydrogenation.
To convert an alkene to an alkane, one hydrogen molecule for every
alkene molecule is required:
CH3 CH3 l l CH3-C=CH2 + H-H CH3-CH-CH3
2-methyl-1-propene hydrogen 2-methylpropane
To convert and alkyne to an alkane, two hydrogen molecules for
every alkyne molecule are required:
CH3-C=CH + 2 H-H CH3-CH2-CH3
(1-)propyne hydrogen propane
Fats and Food
Saturated Fat – contains all single bonds
Monounsaturated Fat - contains one multiple bond
Polyunsaturated Fat - contains more than one multiple bond
What about “hydrogenated” vegetable oils? (saturated)
What are so called “good fats”?
(unsaturated or non-hydrogenated)
Aromatic Hydrocarbons
Aromatic Compounds
Historically: compounds with an odor or aroma
Today: benzene and all other carbon compounds that have benzene-like structures and properties
Benzene’s possible structure intrigued chemists for years, because its properties could not be explained by accepted theories of bonding and reactivity…
Aromatic Hydrocarbons
Properties of benzene
Molecular Formula: C6H6
no evidence of double or triple bonds
all C-C bonds are the same length
all carbons are identical and each one is bonded to one hydrogen
it is very stable
Aromatic Hydrocarbons
August Kekule proposed a cyclic structure for benzene
(1865) inspired by a dream of a snake who grabbed its own tail
disputed because evidence shows all C-C bonds
are the same length, and neither a single or
double bond is present
Aromatic Hydrocarbons
Current benzene model:
based on the resonance theory and the molecular orbital theory)
structure is a hybrid of the 2 structures shown by Kekule
valence electrons are evenly distributed (delocalized) over the entire ring
Naming Aromatics
named as relatives of benzene
One substituent: “alkyl” benzene
CH3 CH2CH3
methylbenzene ethylbenzene
Naming Aromatics
Two substituents: must indicate the location of the substituents
Numbering starts at one of the substituents and goes counterclockwise or clockwise to obtain the lowest possible pair of numbers
CH3 CH3 CH3
CH3
CH3
CH3
1,2-dimethylbenzene 1,3-dimethylbenzene 1,4-dimethylbenzene
ortho meta para
o-dimethylbenzene m-dimethylbenzene p-dimethylbenzene
Naming Aromatics
Larger Molecules: benzene ring is a branch (phenyl group -C6H5)
CH3-CH-CH2-CH2-CH3
2-phenylpentane
Aliphatic vs. Aromatic
Any hydrocarbon in which the bonding capacity of carbon is 4 is considered an aliphatic hydrocarbon All alkanes, cycloalkanes, alkenes and alkynes are
considered aliphatic as the carbon atoms in these molecules all have a bonding capacity of 4.
Any hydrocarbon in which the bonding capacity of carbon is 3 is considered an aromatic hydrocarbon All molecules containing the benzene aromatic ring
are considered aromatic as the carbon atoms in these molecules all have a bonding capacity of 3
Aliphatic or Aromatic ?
A. CH3-CH=CH-CH3
Aliphatic
B. CH2CH3
Aromatic
C. methylpropene
Aliphatic
D. 1,2-dimethylbenzene Aromatic
E. 2-phenylhexane
Aromatic
Reactions of Aromatic
Hydrocarbons Aromatic hydrocarbons can undergo cracking, reforming and combustion
similar to other hydrocarbons
Cracking – larger aromatic hydrocarbons are broken into smaller fragments in the absence of air
CH2-CH2-CH3
Example: + H-H + CH3-CH2-CH3
Reforming – smaller aromatic hydrocarbons combine to form larger ones; CH=CH2
Example: + CH2= CH2 + H-H
Combustion – hydrocarbons burn in the presence of oxygen to form
carbon dioxide and water CH3
Example: + 9 O2 7 CO2 + 4 H2O