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Spring 2014

Lecture 10:Organic Compounds

Course lecturer :

Jasmin Šutković

29 th April 2014

Contents International University of Sarajevo

1. Functional classes and types of Org. CompoundsAlkanesAlkenesAlkynesAromatic compounds

2. Isomeric variations of Organic compounds

3. Reactivity of Organic compound

4. Chemical reactions of Organic Compounds SubstitutionAdditionElimination

Functional Groups and Classes of Organic Compounds

Organic compounds are covalent compounds composed primarily of carbon and hydrogen

Carbon is unique among the elements in its ability to catenate, forming long chains and cyclic structures in a wide variety of compounds

Functional groups are structural units that determine the chemical reactivity of a molecule under a given set of conditions – Can consist of a single atom or a group of atoms

– Organic compounds are classified into several major categories based on the functional groups they contain

Organic Compounds main classes

Organic compounds functional groups

  CnH2n+2 CH4

C2H6

C3H8

C4H10

etc.

C2H6 ethane H H

H—C—C—H

H H

Single bond…

ALKANES

•  sp3 conformation

• bond angles = 109.5o

• σ-bonds (sigma)

Rotation about C--C (conformations)

 

C

H

HH

H

C

HH

C3H8 propane

H C C C H

H

H

H

H

H

H

projection formula

CH3CH2CH3 partially condensed formula

C4H10 butane(s)

H C C C C

H

H

H

H

H

H

H

H

H

H C C C

H

H

C

H

H

H

H

H

H

H

CH3CH2CH2CH3 CH3CHCH3

CH3

projection

partially condensed

stick formulas

Two isomers of butane C4H10:

CH3CH2CH2CH3 n-butane

CH3CH3CHCH3 isobutane

Isomers are different compounds that have the same molecular formula

Alkane name

CH4 methane

C2H6 ethane

C3H8 propane

C4H10 butanes

C5H12 pentanes

C6H14 hexanes

C7H16 heptanes

C8H18 octanes

C9H20 nonanes

C10H22 decanes

…….

C20H42 eicosanes

each new common name requires a new prefix…

hexanes C6H14 common names

CH3CH3CH2CH2CH2CH2CH3 CH3CHCH2CH2CH3 n-hexane isohexane

IUPAC nomenclature (Geneva, 1920)

NAMES OF RADICALS (ALKYL GROUPS):

CH3- “methyl” CH3Cl methyl chloride

CH3OH methyl alcohol, etc.

CH3CH2- “ethyl”

CH3CH2CH2- “n-propyl” CH3CHCH3 “isopropyl” |

CH3CH2CH2Br BrCH2CH2CH3 CH3

CH2CH2Br

n-propyl bromide

CH3CHCH3

BrCH3CHBr

CH3CH3

CHBr

CH3

isopropyl bromide

CH3CH2CH2CH2Cl CH3CH2CH2CH2

ClClCH2CH2CH2CH3

n-butyl chloride

CH3CHCH2CH3

Cl

CH3CH2CHCH3

ClCH3CH2CH

CH3

Cl

sec-butyl chloride

IUPAC rules for naming alkanes:

1. parent chain = longest continuous carbon chain “alkane”.

2. branches on the parent chain are named as “alkyl” groups.

3. number the parent chain starting from the end that gives you the lower number for the first branch (principle of lower number).

4. assign “locants” to the alkyl branches.

5. if an alkyl group appears more than once use prefixes: di, tri, tetra, penta…; each alkyl group must have a locant!

6. the name is written as one word with the parent name last. The names and locants for the alkyl branches are put in alphabetic order (ignore all prefixes except iso) separating numbers from numbers with commas and letters from

numbers with hyphens.

fossil fuels:

natural gas

petroleum

coal

petroleum is a complex mixture of hydrocarbons

1. solvents

2. fuels

3. raw materials for chemical syntheses

separated into fractions by fractional distillation in an oil refinery

Alkenes CnH2n

“unsaturated” hydrocarbons

C2H4 ethylene

Functional group = carbon-carbon double bond

sp2 hybridization => flat, 120o bond angles

σ bond & π bond => H2C=CH2

C CH

H H

H

C3H6 propylene CH3CH=CH2

C4H8 butylenes CH3CH2CH=CH2

α-butylene

1-butene

CH3CH3CH=CHCH3 CH3C=CH2

β-butylene isobutylene

2-butene 2-methylpropene

There are two 2-butenes:

cis-2-butene trans-2-butene

“geometric isomers” (diastereomers)

C C C CH3C

H H

CH3 H3C

H CH3

H

E/Z system is now recommended by IUPAC for the designation of geometric isomerism.

1. Use the sequence rules to assign the higher priority * to the two groups attached to each vinyl carbon.

2. * * *

*

(Z)- “zusammen” (E)- “entgegen”

together opposite

CIS /TRANS or E-Z systems

Nomenclature of alkenes (basic steps):

1. Parent chain = longest continuous carbon chain that contains the C=C.

2. alkane => change –ane to –ene prefix a locant for the carbon-carbon double bond using the principle of lower number.

2. If a geometric isomer, use E/Z (or cis/trans) to indicate which isomer it is.

C CH

H3C CH2CH3

CH3

C CH3C

H Br

Cl

*

* *

*

(Z)-3-methyl-2-pentene

(3-methyl-cis-2-pentene)

(E)-1-bromo-1-chloropropene

Alkynes CnH2n-2

C2H2 H—C C—H sp configuration

Main representative is Acetylene OR Ethyne

Example :

C3H4 CH3CCH propyne OR methyl-acetylene

Nomenclature:

common names: “alkylacetylene”

• IUPAC: parent chain = longest continuous carbon chain that contains the triple bond.

• add -yne

• prefix locant for the triple bond, etc.

CH3CH2CCCH3 2-pentyne

ethylmethylacetylene

“terminal” alkynes have the triple bond at the end of the chain:

CH3CH3CH2CCH HCCCHCH2CH3

1-butyne 3-methyl-1-pentyne

ethylacetylene sec-butylacetylene

Aromatic compounds

Aromatic compounds are compounds which contain a benzene ring in their molecules

Benzene C6H6

Methylbenzene C7H8

Ethylbenzene C8H10

Benzene

The six carbon-carbon bonds in benzene are identical, intermediate in length between double and single bonds

Sigma bonding in benzene

Six carbon atoms joined to form a hexagonal planar ring.

Each carbon has four valence electrons!

One of these is used to form a bond with a hydrogen atom.

Two other electrons are used to form sigma bonds with the carbon atoms on either side.

Range and scope of aromatic chemistry

Pharmaceutical compounds, e.g. Morphine

Herbicides (Herbicides, also commonly known as weedkillers, are pesticides used to kill unwanted plants)

Detergents (e.g. Sodium dodecylbenzenesulfonate- It is a major component of laundry detergent )

Dyes

Aromatic compounds and cancer

Some aromatic compounds are carcinogenic, e.g. Benzene

However, not all aromatic compounds are carcinogenic; aspirin is an example

1. First family is the hydrocarbons, which include alkanes, with the general molecular formula CnH2n+2 where n is an integer; alkenes represented by CnH2n; alkynes represented by CnH2n–2; and arenes (CnHn)

2. Second family is the halogen-substituted alkanes, alkenes, and arenes, which include the alkyl halides and aryl halides

Organic compounds groups :

3. Third family is the oxygen-containing organic compounds,

which are divided into two main types: a. Those that contain at least one C–O single bond, which

include alcohols, phenols, and ethers

b. Those that contain a carbonyl group (> CO), which include aldehydes, ketones, and carboxylic acids

4. Fourth family is the carboxylic acid derivatives; these are compounds in which the H atom on the –CO2H functional group is replaced either by an alkyl group, producing an ester, or by an amine, forming an amide

5. Fifth family is the nitrogen-containing organic compounds; these include amines, nitriles (which have a CN triple bond) and nitro compounds (which contain the NO2 group)

Isomeric Variations in Structure

• Isomers are different compounds that have the same molecular formula

• Three main types of isomers: 1. Conformational

2. Constitutional (structural)

3. Stereoisomers

Conformational Isomers

• The C–C single bonds in alkanes are formed by the overlap of an sp3 hybrid orbital on one carbon atom with an sp3 hybrid orbital on another carbon atom, forming a bond (sigma)

• Differences in three-dimensional structure resulting from rotation about a bond are called differences in conformation, and each different arrangement is called a conformational isomer

• Differences between the conformations are depicted in drawings called Newman projections

– A Newman projection represents the view along a C–C bond axis, with the carbon that is in front shown as a point and the carbon that is bonded to it shown as a circle

– In one extreme, the eclipsed conformation, the C–H bonds on

adjacent carbon atoms are parallel and lie in the same plane

– In the other extreme, the staggered conformation, the hydrogen atoms are positioned as far from one another

as possible

Conformational Isomers

• Newman projections are useful for predicting the stability of conformational isomers

– The eclipsed conformation is higher in energy than the staggered conformation because of electrostatic

repulsions between hydrogen atoms

– The staggered conformation is the most stable because electrostatic repulsion between the hydrogen atoms on

adjacent carbons is minimized

Newmans projections

Constitutional (Structural) Isomers

• Constitutional (structural) isomers differ in the connectivity of the atoms – The two alcohols, 1–propanol and 2–propanol, have the same

molecular formula (C3H8O), but the position of the –OH group differs, which causes differences in their physical and

chemical properties

• In the conversion of one constitutional isomer to another, at least one bond must be broken and reformed at a different position in the molecule

Propanol…

Stereoisomers

• Stereoisomers are molecules that have the same connectivity but whose component atoms have different orientations in space

• Two types of stereoisomers:

1. Geometric isomers differ in the relative placement of substituents in a rigid molecule; members of an isomeric pair are either cis or

trans, with interconversion between the two forms requiring breaking and reforming one or more bonds; their structural differences causes them to have different physical and chemical properties and to exist as two distinct chemical compounds

Stereoisomers

2. Optical isomers are molecules that are mirror images but cannot be superimposed on one another in any orientation

a. Optical isomers have identical physical properties, although their chemical properties may differ

b. Molecules that are nonsuperimposable mirror images of each other are said to be chiral; an achiral object is one that can be superimposed on its mirror image

Stereoisomers

Stereoisomers

• Most organic molecules that are chiral have at least one carbon atom that is bonded to four different groups – This carbon is designated by an asterisk in structural drawings

and is called a chiral center, chiral carbon atom, asymmetric carbon atom, stereogenic center, or stereocenter

• A molecule and its non-superimposable mirror image are called enantiomers!

Stereoisomers

Stereoisomers

• Interactions of enantiomers with other chiral molecules

In living organisms, every molecule with a stereocenter is found as a single enantiomer, not a mixture

At the molecular level, our bodies are chiral and interact differently with the individual enantiomers of a particular compound

Only one enantiomer of a chiral substance interacts with a particular receptor, initiating a response; the other enantiomer may not bind at all, or it may bind to another receptor, producing a different response

Cis and Trans conformations

The reactivity of a molecule is affected by the degree of substitution of the carbon bonded to a functional group; the carbon is designated as primary, secondary, or tertiary

– Primary carbon is bonded to only one other carbon and a functional group

– A secondary carbon is bonded to two other carbons and a functional group

– A tertiary carbon is bonded to three other carbons and a functional group

Reactivity of Organic Molecules

Common Classes of Organic Reactions

Five common types of organic reactions:

1. Substitution 2. Elimination 3. Addition 4. Free-radical reactions 5. Oxidation-reduction reactions

Substitution

In a substitution reaction, one atom or group of atoms in a substance is replaced by another atom or group of atoms from another substance

A typical substitution reaction is the reaction of hydroxide ion with methyl chloride

CH3Cl + OH– CH3OH + Cl–

Methyl chloride has a polar C–Cl bond, so the carbon atom has a partial positive charge

Electronegative Cl atom is replaced by another electronegative species that is a stronger nucleophile, OH–

Elimination

Reactions in which adjacent atoms are removed, or “eliminated,” from a molecule with the formation of a multiple bond and a small molecule are called elimination reactions

General form:

Addition

A reaction in which the components of a species A–B are added to adjacent atoms across a carbon-carbon multiple bond is called an addition reaction

An example is the reaction of HCl with ethylene to give chloroethane:

HCl + CH2CH2 CH3CH2Cl

Free-Radical Reactions

Many important organic reactions involve free radicals, and the best known is the reaction of a saturated hydrocarbon with a halogen:

CH3CH3 + Br2 CH3CH2Br + HBr

• Free radical reactions occur in three stages: initiation, propagation, and termination

At high temperature or in the presence of light, the weak Br–Br bond is broken in an initiation step that produces a number of Br atoms

During propagation, a bromine atom attacks ethane, producing a free radical, which then reacts with another bromine molecule to produce ethyl bromide; the sum of the propagation steps corresponds to the overall balanced equation for the reaction

Three possible termination steps: the combination of two bromine atoms, of two ethyl radicals, or of an ethyl and a bromine radical

HALOGEN ELEMENTS

Are a group in the periodic table consisting of five chemically related elements, fluorine (F), chlorine(Cl),  bromine (Br), iodine (I), and astatine (At).

In the modern IUPAC nomenclature, this group is known as group 17 in PSE

Oxidation-Reduction Reactions

Oxidation-reduction reactions are common in organic chemistry and can be identified by changes in the number of oxygens at a particular position in the hydrocarbon skeleton or in the number of bonds between carbon and oxygen at that position

General Properties and Reactivity of Functional Groups

The functional groups characteristic of each class of organic compounds determine the general properties and reactivity of that class

Readings…

Follow the slides and the book chapter 24 named ORGANIC COMPOUNDS

http://www.chemguide.co.uk/basicorg/conventions/names.html

This link is for additional tutorial for naming of simple organic compound…

Two more lectures to go…