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.