1 Introduction to Organic Chemistry
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Transcript of 1 Introduction to Organic Chemistry
Organic Compounds
Alkanes
Introduction to Organic ChemistryAlkanesAlkanes Nomenclature SystemStructural IsomersCycloalkanesReactions of Alkanes and CycloalkanesAlkyl Halides
Importance of carbonBasis 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 chemicals are possible.
Allotropic forms of carbon
Elemental carbon exists in 4 forms.Amorphous- no definite structure
Graphite- layered sheets of rings
Diamond- tetrahedral
Spherical- combination of rings
We commonly look at organic species by class.
Alkanessimplest class of organic compoundscontain only carbon and hydrogenonly have single bonds
Classes of organic compounds
H C
H
H
H
H C
H
C
H
H
H
H
H C
H
C
H
C
H
H
H
H
H H C
H
C
H
C
H
H
H
H
C H
H
H
methane ethane propane butane
Line representationsMolecular Formula C5H12
Possible line representationsCH3CH2CH2CH2CH3 n-pentane
CH3CH(CH3)CH2CH3 2-methylbutane
(CH3)4C 2,2-dimethylpropane
All are structural isomers of C5H12.
This system is used to show the arrangement of atoms in a molecule in a single line.
Structural isomersCompounds with the same number of atoms and bonds but with different arrangements.
CH3CH(CH3)CH2CH3
H3CCH
CH2
CH3
CH3
(CH3)4C
H3C C
CH3
CH3
CH3
CH3CH2CH2CH2CH3
H3C
H2C
CH2
H2C
CH3
FormulaStructural Formula
• Shows all atoms with bonds.• Bonds are represented as lines.
Example. Propane
Condensed formula• A type of line format.• Shorthand way of writing formula.
• Lists all atoms in order and tells how they are bound together.
Example Propane CH3CH2CH3
H C
H
C
H
C
H
H
H
H
H
Shapes of organic molecules
Properties of organic compounds are related to their geometry.
Bond order will help to determine geometry.• single bond = bond order of 1
• double bond = bond order of 2• triple bond = bond order of 3
Models are useful to help see these shapes.
ModelsThree dimensional representations
Ball and stick
Space filling
Ethane• Bond order = 1• Single C-C bond• Tetrahedral geometry
Hybridization
• The reason is that the s and p orbitals will blend together, giving us 4 equivalent orbitals.
• The s and p orbitals are very close in energy so the four electrons spend time in the s and the 3 different p.
• This process is called hybridization.
You should be asking . . . . “ If carbon has two s electrons and two p electrons, how do we get a tetrahedral shape? ”
Hybridization
In the case of a carbon that has 4 single bonds, all of the orbitals are hybrids.
sp325% s and 75% p character
+ 3
s p sp3
1 4
Ethane
CC
1s orbital of H
! bond
sp3
hybrids
We’ll look at other hybridsin the next chapter.
Ethene
• Bond order = 2• Double C-C bond - sp2 hybrid• Planer geometry
• Bond order = 3• Triple C-C bond - sp hybrid• Linear geometry
Ethyne
Bond length and strength
bond lengthdistance between two bonded atoms.single bonds > double bonds > triple bonds
bond strengthenergy necessary to break a chemical bond.triple bonds > double bonds > single bonds
Classifying organic compounds
Classify compounds by functional group.
Functional groupSpecific combination of atoms that gives a known type of behavior
Hydrocarbons C and H onlyAlcohols R-OHOHAcids R-COOHCOOHAmines R-NHNH22Ketones R(C=OC=O)R’Aldehydes R-CHOCHO
R, RR, R’’ = any hydrocarbon chain. = any hydrocarbon chain.
AlkanesSimplest members of the hydrocarbon family.
• contain only hydrogen and carbon• only have single bonds
All members have the general formula ofCnH2n+2
First four members of the alkanes
Name # of C Condensed formulaMethane 1 CH4
Ethane 2 CH3CH3
Propane 3 CH3CH2CH3
Butane 4 CH3CH2CH2CH3
Called a homologous serieshomologous series“Only differ by number of CH2 groups”
Alkanes
Physical PropertiesNon-polar moleculesNot soluble in waterLow densityLow melting pointLow boiling point
These go upas the number
of carbonsincrease.
These go upas the number
of carbonsincrease.
Alkanes
Name BP (oC) MP (oC) DensityMethane -161.7 -182.6 0.424Ethane - 88.6 -172.0 0.546Propane - 42.2 -187.1 0.582Butane -0.5 -135.0 0.579Pentane 36.1 -129.7 0.626Hexane 68.7 - 94.0 0.659Heptane 98.4 - 90.5 0.684Octane 125.6 - 56.8 0.703 Nonane 150.7 -53.7 0.718Decane 174.0 -29.7 0.730
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• Where functional groups are (if any)
The IUPAC Nomenclature System provides a uniform set of rules that we can follow.
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. Number the location of each branch.
5. List multiple branches alphabetically - the di, tri, ... don’t count..
Base namesPrefix
Length of carbon chain
Meth 1
Eth 2
Prop 3
But 4
Pent 5
Hex 6
Hept 7
Oct 8
Non 9
Dec 10
Prefixes are derived from Greek (except for ‘Non’ which is from Latin.
Go figure?
Naming alkanes
Six carbon atoms in the longest chainuse base name of hex
All carbon and hydrogen with single bondsuse ane ending
Name of compound is hexane
C - C - C - C - C - C
Longest C chain has 4 carbon - use but
All C and H with single bonds - use ane
Parent name: butane
CH3- on 2nd carbon - methyl
2-methylbutane
H3CCH
CH2
CH3
CH3 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
3,5-dimethyl heptane
3-ethyl-5-methylheptane
2,3,3,7,8-pentamethyldecane
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 - it may help.
Another example
(CH3)2CHCH2CH2CH(CH3)2
C C | |C - C - C - C - C - C
Now name it!
Another example
1. Longest chain is 6 - hexane
2. Two methyl groups - dimethyl
3. Use 2,5-dimethylhexane
C C | |C - C - C - C - C - C
Even more examples
3-ethyl-2,2-dimethylhexane
4-propylheptane
Even more examples
4-methyldecane
2,2,4,4,6-pentamethylheptane
Cycloalkanes
C - C | |
C - Ccyclobutane
C - C / \
C C \ /
C - C
cyclohexane
C - C \ /
Ccyclopropane
Cyclic alkanes of the general formula, CnH2n.
Named as the parent alkane but add cyclocyclo--
Cyclic alkanes of the general formula, CnH2n.
Named as the parent alkane but add cyclo-
• When they occur as a branch, use the same rules as with any branch.
• Cyclohexane as a branch would be cyclohexyl.
Cycloalkanes.
ethylcyclopentane
cycloheptane
1-cyclohexylheptane
cis and trans isomers• The book discusses cis and trans isomers. We’ll
worry about it in the next chapter. Just a couple of examples for now.
HCH2C
CH2
CHCH2
H2C
CH3
CH3
CH
H2C
CH2
CHCH2
H2C
CH3
H3C
cyclohexane trans-1,4-dimethylcyclohexane
cis-1,4-dimethylcyclohexane
Sources of alkanesAlkanes can be obtained by refinement orhydrogenation of:
petroleumpetroleumshale oilshale oilcoalcoal
Low molecular weight alkanes can be obtained directly from natural gas.
Reactions of alkanesCombustionCH4(g) + 2 O2(g) CO2(g) + 2H2O(g)Many alkanes are used this way - as fuels
Methane - natural gasPropane - used in gas grillsButane - lightersGasoline - mixture of hydrocarbons
Reactions of alkanesHalogenation
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 - used as anesthesia
• 1,2-dichloroethane - dry cleaning fluid
This is a substitution reaction
Alkyl halides
Structure and naming aresimilar to the alkanes.
Theses are alkanes with one or morehalogens in place of hydrogen.
General formula R - X
Any alkanechain
A halogenF, Cl, Br, I
Naming alkyl halides
The names used for halogens are
Halogen Name Symbol fluorine fluoro -F chlorine chloro -Cl bromine bromo -Br iodine iodo -I
Naming alkyl halides
!Follow the same system aswith alkanes.
!Give the name and carbonnumber for the halide justlike a side branch.
C - C - F C - C - C C-C-C-C-C | |
Cl C-Br
1-fluorofluoroethane
2-chlorochloropropane
1-bromobromo-2-methylpentane
Nomenclature examples
CH3CHBrCH32-bromopropane
CH3CH2C(CH3)ClCH2CH2CH33-chloro-3-methylhexane
CCl2F2dichlorodifluoromethane
CH3C(CH3)ICH32-iodo-2-methylpropane
Examples
bromochloromethane
1-bromo-3-chloropropane
(chloromethyl)cyclopropane
Properties of alkyl halides
Melting points and Boiling points• Higher than alkanes.• Increase with MW and number of halides.
Solubility• Not soluble in water.• Soluble in nonpolar solvents like alkanes
other alkyl halides.
Classification of alkyl halides
Based on the number of alkyl groups attached to the carbon.
Results in four classes - primary - secondary - tertiary
- quaternary
This is true for other functional groups as well
Classification of alkyl halides
R - CC - X
Primary
R |R - CC - R | X
Tertiary R |R - CC - R | R
Quaternary
R - CC - R | X
Secondary
Reactions of alkyl halides
Principal reaction type is halide exchange.halide exchange.- a type of substitution reactionsubstitution reaction
halide exchange
CH3-Br + I- CH3-I + Br-
CH3-Br + OH- CH3-OH + Br-
Note!Note!Does not work with tertiary alkyl halides.