Organic Chemistry

1
Organic Chemistry

• Organic Compounds
• Hydrides of Carbon
• Stereoisomers
• Alkanes
• Nomenclature
• Functional Groups
• SN2 Mechanism
• Synthesis
• Polymers

2
The importance oforganic chemistry

• Many areas rely on organic chemistry, including
• Biology
• Petroleum
• Polymers
• Genetic Engineering
• Agriculture
• Pharmacology
• Consumer Products

3
Importance of carbon

• Basis for all life.
• Form stable covalent bonds to other carbon atoms
  - catenation.
• Can form single, double and triple bonds.
• Long carbon chains can be produced.
• Will bond with many other elements.
• A HUGE number of compounds is possible.

4
Hydrides of carbon

• Catenation
• The formation of chains of atoms of the same
  element.
• This key feature of carbon permits a vast number
  of compounds to exist.
• One simple class of compound is the alkane which
  has only C, H and single bonds.
• methane ethane propane
  butane

5
Formulas and models

• Organic molecules can have very complex
  structures.
• A number of formats are used to represent
  organic compounds.
• Each has its own advantages but the goal is the
  same, to accurately describe the structure of a
  compound.
• Lets look at some different representations.

6
Formula

• Condensed structural formula
• Shorthand way of writing formula.
• Lists all atoms in order and tells how they
• are bound together.
• Example. Propane
• CH3CH2CH3
• This is a convenient format for describing a
  molecule using text.

7
Constitutional isomers

• Compounds with the same number and type of atoms
  but with different arrangements.
• Molecular Formula C5H12
• Condensed structural formulas.
• CH3CH2CH2CH2CH3 pentane
• CH3CH(CH3)CH2CH3 2-methylbutane
• (CH3)4C 2,2-dimethylpropane
• All are constitutional isomers of C5H12.

8
Line formula

• Similar to structural formula.
• Each line represents a bond.
• Carbons are assumed to be present at the end of
  each line segment.
• Hydrogen is not shown when bound to carbon.

9
Models

• Three dimensional representations

Ball and Stick
Space Filling
Both are models of propane.
10
Multiple bonds

• Another key feature of carbon is its ability to
  form double and triple bonds.
• This can be between two carbons
• alkenes (CC) and alkynes (C C)
• It can also be between carbon and another
  element.
• CO
• CN-
• C N

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Ethane, C-C single bond
12
Ethene or ethylene, C-C double bond
13
Ethyne or acetylene, C-C triple bond
14
Stereoisomers

• Constitutional isomers are not the only types
  that can exist.
• Stereoisomers have
• the same order and types of bonds.
• different spatial arrangements.
• different properties.
• Many biologically important compounds, like
  sugars, exist as stereoisomers. Your body can
  tell the difference.

15
Stereoisomers

• Two kinds of stereoisomers exist.
• Cis-trans isomers
• When a double bond exists between carbons or
  carbons form a ring, a molecule can exist in two
  geometric forms.
• Optical isomers
• When molecules can exist as mirror-image isomers
  or enantiomers..

16
Geometric isomers
There are two possible arrangements. Example
2-butene
H
H3C
CH3
H3C
CC
CC
H
H
CH3
H
cis Largest groups are on the same side.
trans Largest groups are on opposite sides.
17
Retinal and sight
cis-retinal
Light causes a change from cis- to trans-. This
is how we see. Several enzymes are required to
convert trans-retinal Back to the cis-form.
trans-retinal
18
3-D models of retinal
cis-
trans-
19
Enantiomers

• Pairs of stereoisomers
• Sometimes designated by D- or L- at the start of
  the name.
• They are mirror images that cant be
  superimposed.

If you dont believe it, give it a try!
20
Enantiomers
21
L- and D- glyceraldehyde
22
Enantiomers

• Stereocenter.
• Chiral center or asymmetric carbon - four
  different things are attached to it.
• Cl
• I - C - F
• Br
• A molecule that has one stereocenter exists as a
  pair of enantiomers.

Chiral center
23
Examples

• Is the red carbon a stereocenter?

H CO
H-C-OH CH2OH
24
Physical properties

• Optical activity
• ability to rotate plane-polarized light.
• dextrorotatory - rotate clockwise
• - use symbol
• - usually D isomers
• levorotatory - rotate counterclockwise
• - use - symbol
• - usually L isomers

25
Plane polarized light
Light is passed through a polarized filter. A
solution of an optical isomer will rotate the
light one direction.
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Alkanes

• Simplest members of the hydrocarbon family.
• contain only hydrogen and carbon
• only have single bonds
• All members have the general formula of
• CnH2n2

Twice as many hydrogen as carbon 2
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Alkanes

• First four members of the alkanes
• Name of C Condensed formula
• Methane 1 CH4
• Ethane 2 CH3CH3
• Propane 3 CH3CH2CH3
• Butane 4 CH3CH2CH2CH3
• Called a homologous series
• Members differ by number of CH2 groups

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Alkanes

• Physical Properties
• Nonpolar molecules
• Not soluble in water
• Low density
• Low melting point
• Low boiling point

These go up as the number of carbons increases.
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Alkanes

• Name bp, oC mp, oC
  Density at 20 oC
• Methane -161.7 -182.6 0.000 667
• Ethane - 88.6 -182.8
  0.001 25
• Propane - 42.2 -187.1 0.001 83
• Butane -0.5 -135.0 0.002
  42
• Pentane 36.1 -129.7 0.626
• Hexane 68.7 - 94.0
  0.659
• Heptane 98.4 - 90.5 0.684
• Octane 125.6 - 56.8 0.703
  
• Nonane 150.7 -53.7 0.718
• Decane 174.0 -29.7 0.730

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Sources of alkanes
Alkanes can be obtained by refining
or hydrogenation of petroleum shale
oil coal Low molecular mass alkanes can be
obtained directly from natural gas.
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Reactions of alkanes

• Combustion
• CH4(g) 2O2(g) CO2(g) 2H2O(g)
• Many alkanes are used this way - as fuels
• Methane - natural gas
• Propane - used in gas grills
• Butane - lighters
• Gasoline - mixture of many hydrocarbons, not
  all alkanes

32
Reactions of alkanes

• Halogenation
• A reaction where a halogen replaces one or more
  hydrogens.
• CH4(g) Cl2(g)
  CH3Cl(g) HCl(g)
• Used to prepare many solvents
• dichloromethane - paint stripper
• chloroform - once used as anesthesia
• 1,2-dichloroethane - dry cleaning fluid

heat or light
33
Organic nomenclature

• Organic molecules can be very complex.
• Naming system must be able to tell
• Number of carbons in the longest chain
• The location of any branches
• Which functional groups are present and where
  they are located.
• The IUPAC Nomenclature System provides a uniform
  set of rules that we can follow.

34
Naming alkanes

• 1 Find the longest carbon chain. Use as
  base name with an ane ending.
• 2 Locate any branches on chain. Use base names
  with a yl ending.
• 3 For multiple branch of the same type,
  modify name with di, tri, ...
• 4 Show the location of each branch with
  numbers.
• 5 List multiple branches alphabetically - the
  di, tri, ... dont count..

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Base names

• Prefix Carbons
• Meth- 1
• Eth- 2
• Prop- 3
• But- 4
  
• Pent- 5
• Hex- 6
• Hept- 7
• Oct- 8
• Non- 9
• Dec- 10

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Naming alkanes

• Lets practice some names
• We will be leaving out the hydrogens.
• This makes the carbon chain easier to see.

37
Naming alkanes
C - C - C - C - C - C

• 1. 6 carbon in longest chain - use base
  name of hex
• 2. All carbon and hydrogen with single bonds
• - use ane ending
• 3. Name of compound is hexane

38
Naming alkanes
C
C - C - C - C - C

• 1. Longest C chain has 5 carbon - use pent
• 2. All C and H with single bonds - use
  ane Parent name pentane
• 3. CH3- on 2nd carbon - methyl
• 2-methylpentane

39
Examples
C-C-C-C-C-C C-C C
C-C-C-C-C-C-C C-C C
C-C-C-C C-C-C
C-C-C-C-C-C-C
C
40
Examples
C-C-C-C-C-C C-C C
C-C-C-C-C-C-C C-C C
3,5-dimethylheptane
C-C-C-C C-C-C
C-C-C-C-C-C-C
C
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Examples
C-C-C-C-C-C C-C C
C-C-C-C-C-C-C C-C C
3,5-dimethylheptane
3-ethyl-5-methylheptane
C-C-C-C C-C-C
C-C-C-C-C-C-C
C
42
Examples
C-C-C-C-C-C C-C C
C-C-C-C-C-C-C C-C C
3,5-dimethyl heptane
3-ethyl-5-methylheptane
C-C-C-C C-C-C
C-C-C-C-C-C-C C
2,3,3,7,8-pentamethyldecane
43
Another example
Name the following.
(CH3)2CHCH2CH2CH(CH3)2
This is a condensed structural formula. First
convert it to a carbon skeleton, leaving out the
hydrogens.
44
Another example
(CH3)2CHCH2CH2CH(CH3)2
C C
C - C - C - C - C - C
Now name it!
45
Another example
C C
C - C - C - C - C - C
1. Longest chain is 6 - hexane 2. Two methyl
groups - dimethyl 3. Use 2,5-dimethylhexane
46
Classifyingorganic compounds

• Classify compounds by functional group.
• Functional group Specific combination of
  atoms that gives a known type of
  behavior.
• Examples
  
• Hydrocarbons C and H only
• Alcohols R-OH
• Acids R-COOH
• Amines R-NH2
• Ketones R(CO)R
• Aldehydes R-CHO

47
Nomenclature of compounds containing functional
groups

• The IUPAC system deals with functional groups two
  different ways.
• Modification of the hydrocarbon name to indicate
  the presence of a functional group.
• Alcohol, -OH use -ol ending.
• Aldehyde, -CHO use -al ending.
• Acid, -COOH use -oic acid ending.
• Treat the group as a branch.
• Halogens, amines and ethers

48
Alcohol example
C - C - C - C - O - H
Base contains 4 carbon - alkane name is
butane - remove -e and add -ol alcohol name -
butanol OH is on the first carbon so
- 1-butanol
49
Acid example

• Example CH3CH2COOH
• 1. Longest chain containing carbonyl is 3,
• propane
• 2. The -e ending is replaced with -oic acid,
• propanoic acid

50
Naming alkyl halides

• 1. Follow the same system as with alkanes.
• 2. Give the name and carbon number for the
  halide just like a side branch.

C - C - F C - C - C C-C-C-C-C

Cl C-Br
1-fluoroethane
2-chloropropane
1-bromo-2-ethylpentane
51
SN2 mechanism

• Reaction mechanisms provide a powerful way to
  organize the vast amount of information about
  organic reactions.
• SN2 mechanism
• One very important reaction mechanism.
• The symbol (SN2) stands for substitution
  nucleophilic bimolecular.
• Nucleophile
• Nucleus loving. A species that is attracted
  by a positive charge.

52
SN2 mechanism

• Example
• HO- CH3Br (aq) CH3OH (aq) Br-
• For this reaction
• HO- is the nucleophile.
• CH3Br is the substrate - a species that undergoes
  reaction.
• Cl- is the leaving group. Because it is replaced
  by HO-.

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SN2 mechanism

• The mechanism takes place in a single step. This
  is supported by the observed rate law.
• Rate k HO-CH3Br
• SN2 reactions also take place with inversion of
  configuration.

54
SN2 mechanism
d
d-
To account for the inversion, the nucleophile
must approach from the back of the carbon The
nucleophile acts as a Lewis base and the
substrate as a Lewis acid.
d-
d
55
SN2 mechanism
56
SN2 mechanism

• Predicting whether an SN2 reaction will occur is
  possible.
• The SN2 reaction
• Nuc- RX RNuc X-
• is similar to a Bronsted-Lowry acid base
  reaction
• B- HX HB X-

57
SN2 mechanism

• To predict whether a SN2 reaction will occur, you
  must consider the relative base strength of the
  nucleophile and the leaving group.
• If the nucleophile is a stronger base, the
  reaction will occur.
• Relative base strength
• OH- gt Cl- gt Br- gt I-

58
Polymers

• Because the monomer units in synthetic polymers
  are all the same (except for copolymers), an
  abbreviated formula can be used.
• Monomer Polymer
• Ethylene polyethylene
• Vinyl chloride poly(vinyl chloride)

59
Other polymer examples

• Formula
• Name Monomer Polymer
• Polypropylene CH3CHCH2 CH-CH2
• Polystyrene -CHCH2 CH-CH2
• Polychloroprene H2CCHCCH2 CH2CHCCH2

( )
CH3
( )
( )
Cl
Cl
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Polymerization

• Polymers are formed either by
• Chain polymerization
• A multi-step process involving initiation,
  propagation and termination. Polymer size is
  relatively uniform.
• Stepwise polymerization
• A process where polymer size and amount increase
  as a function of time.

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Chain polymerization

• Chain initiation
• This is the first step in chain polymerization.
• An initiator is added to form a radical species
  which adds to a monomer.
• The resulting species is also a radical.

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Chain polymerization

• Chain propagation
• The newly formed radical is then able to react
  with another monomer unit.
• This process will continue, resulting in an
  increasing longer chain.

Rad-CH2-CHCl CH2 CHCl
Rad-CH2-CHCl -CH2-CHCl
63
Chain polymerization

• Chain termination
• This occurs when two radicals combine.
• Initially unlikely occurrence because much more
  monomer than radical is present.
• As the monomer is depleted, termination becomes
  much more likely.

Rad-CH2-CHCl -CH2-CHCl Rad-CH2-CHCl -CH2-CHCl
Rad-CH2-CHCl -CH2-CHCl
CHCl-CH2-CHCl-CH2-Rad
64
Step polymerization

• For step polymerization to occur, each monomer
  unit must have two reactive groups.
• Example. Preparation of nylon.

Adipoyl chloride
hexamethylenediamine

H2N(CH2)6NH2
65
Step polymerization

• Product still has two reactive groups.
• Polymer length is a function of time.

66
Polymer types

• Fibers
• These result when the intermolecular forces
  between polymer molecules are strong.
• Chains can be lined up by stretching.

67
Polymer types

• Elastomers
• In these polymers, intermolecular attractions
  are weak.
• Crosslinking the chains is one way of helping
  them to maintain a shape.

68
Polymer types

• Plastics
• Somewhere between fibrous polymers and
  elastomers.
• Thermoplastic
• Soften when heated
• Example - polyethylene.
• Thermoset plastic
• Do not soften when heated.
• These are typically highly crosslinked
  polymers.