AN INTRODUCTION TO ORGANIC CHEMISTRY

AN INTRODUCTION TO ORGANIC CHEMISTRY
A guide for A level students

INTRODUCTION
This Powerpoint show is one of several produced to help students understand selected topics at AS and A2 level Chemistry. It is based on the requirements of the AQA and OCR specifications but is suitable for other examination boards.
Individual students may use the material at home for revision purposes or it may be used for classroom teaching if an interactive white board is available.
Accompanying notes on this, and the full range of AS and A2 topics, are available from the KNOCKHARDY SCIENCE WEBSITE at...
www.knockhardy.org.uk/sci.htm

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CONTENTS
Scope of organic chemistry
Special nature of carbon
Types of formulae
Homologous series
Functional groups
Nomenclature
Investigating molecules
Revision check list

Organic chemistry is the study of carbon compounds. It is such a complex branch of chemistry because...


• CARBON ATOMS FORM STRONG COVALENT BONDS TO EACH OTHER


• THE CARBON-CARBON BONDS CAN BE SINGLE, DOUBLE OR TRIPLE


• CARBON ATOMS CAN BE ARRANGED IN STRAIGHT CHAINS
BRANCHED CHAINS
and RINGS


• OTHER ATOMS/GROUPS OF ATOMS CAN BE PLACED ON THE CARBON ATOMS


• GROUPS CAN BE PLACED IN DIFFERENT POSITIONS ON A CARBON SKELETON

CATENATION is the ability to form bonds between atoms of the same element.
Carbon forms chains and rings, with single, double and triple covalent bonds, because it is able to FORM STRONG COVALENT BONDS WITH OTHER CARBON ATOMS

Carbon forms a vast number of carbon compounds because of the strength of the C-C covalent bond. Other Group IV elements can do it but their chemistry is limited due to the weaker bond strength.

BOND ATOMIC RADIUS BOND ENTHALPY
C-C 0.077 nm +348 kJmol-1


Si-Si 0.117 nm +176 kJmol-1



The larger the atoms, the weaker the bond. Shielding due to filled inner orbitals and greater distance from the nucleus means that the shared electron pair is held less strongly.

CHAINS AND RINGS

CARBON ATOMS CAN BE ARRANGED IN


STRAIGHT CHAINS



BRANCHED CHAINS





and RINGS

MULTIPLE BONDING AND SUBSTITUENTS

CARBON-CARBON COVALENT BONDS CAN BE SINGLE, DOUBLE OR TRIPLE

MULTIPLE BONDING AND SUBSTITUENTS

CARBON-CARBON COVALENT BONDS CAN BE SINGLE, DOUBLE OR TRIPLE





DIFFERENT ATOMS / GROUPS OF ATOMS CAN BE PLACED ON THE CARBONS

The basic atom is HYDROGEN but groups containing OXYGEN, NITROGEN, HALOGENS and SULPHUR are very common.




CARBON SKELETON FUNCTIONAL CARBON SKELETON FUNCTIONAL
GROUP GROUP



The chemistry of an organic compound is determined by its FUNCTIONAL GROUP

MULTIPLE BONDING AND SUBSTITUENTS

ATOMS/GROUPS CAN BE PLACED IN DIFFERENT POSITIONS ON A CARBON SKELETON

MOLECULAR FORMULA C4H10
The exact number of atoms of each
element present in the molecule


EMPIRICAL FORMULA C2H5
The simplest whole number ratio
of atoms in the molecule


STRUCTURAL FORMULA CH3CH2CH2CH3 CH3CH(CH3)CH3
The minimal detail using conventional
groups, for an unambiguous structure there are two possible structures


DISPLAYED FORMULA
Shows both the relative placing of atoms
and the number of bonds between them

SKELETAL FORMULA
A skeletal formula is used to show a simplified organic formula by removing hydrogen atoms from alkyl chains, leaving just a carbon skeleton and associated functional groups

SKELETAL FORMULA
A skeletal formula is used to show a simplified organic formula by removing hydrogen atoms from alkyl chains, leaving just a carbon skeleton and associated functional groups











GENERAL FORMULA
Represents any member of for alkanes it is... CnH2n+2
a homologous series possible formulae... CH4, C2H6 .... C99H200


The formula does not apply to cyclic compounds such as cyclohexane is C6H12
- by joining the atoms in a ring you need fewer H’s

A series of compounds of similar structure in which each member differs from the next by a common repeating unit, CH2. Series members are called homologues and...

all share the same general formula.
formula of a homologue differs from its neighbour by CH2. (e.g. CH4, C2H6, ... etc )
contain the same functional group
have similar chemical properties.
show a gradual change in physical properties as molar mass increases.
can usually be prepared by similar methods.

CH3OH C2H5OH C3H7OH
METHANOL ETHANOL PROPAN-1-OL

GROUP ENDING GENERAL FORMULA EXAMPLE
ALKANE - ane RH C2H6 ethane
ALKENE - ene C2H4 ethene
ALKYNE - yne C2H2 ethyne
HALOALKANE halo - RX C2H5Cl chloroethane
ALCOHOL - ol ROH C2H5OH ethanol
ALDEHYDE -al RCHO CH3CHO ethanal
KETONE - one RCOR CH3COCH3 propanone
CARBOXYLIC ACID - oic acid RCOOH CH3COOH ethanoic acid
ACYL CHLORIDE - oyl chloride RCOCl CH3COCl ethanoyl chloride
AMIDE - amide RCONH2 CH3CONH2 ethanamide
ESTER - yl - oate RCOOR CH3COOCH3 methyl ethanoate
NITRILE - nitrile RCN CH3CN ethanenitrile
AMINE - amine RNH2 CH3NH2 methylamine
NITRO nitro- RNO2 CH3NO2 nitromethane
SULPHONIC ACID - sulphonic acid RSO3H C6H5SO3H benzene sulphonic acid
ETHER - oxy - ane ROR C2H5OC2H5 ethoxyethane

Draw legitimate structures for each molecular formula and classify each one according to the functional group present. Not all the structures represent stable compounds.

carbon atoms have 4 covalent bonds surrounding them
oxygen atoms 2
nitrogen atoms 3
hydrogen 1
halogen atoms 1


C2H6 ONE
C3H7Br TWO
C4H8 FIVE - 3 with C=C and 2 ring compounds with all C-C’s
C2H6O TWO - 1 with C-O-C and 1 with C-O-H
C3H6O SIX - 2 with C=O, 2 with C=C and 2 with rings
C2H7N TWO
C2H4O2 SEVERAL - Only 2 are stable
C2H3N TWO

Draw legitimate structures for each molecular formula and classify each one according to the functional group present. Not all the structures represent stable compounds.

carbon atoms have 4 covalent bonds surrounding them
oxygen atoms 2
nitrogen atoms 3
hydrogen 1
halogen atoms 1


C2H6 ONE
C3H7Br TWO
C4H8 FIVE - 3 with C=C and 2 ring compounds with all C-C’s
C2H6O TWO - 1 with C-O-C and 1 with C-O-H
C3H6O SIX - 2 with C=O, 2 with C=C and 2 with rings
C2H7N TWO
C2H4O2 SEVERAL - Only 2 are stable
C2H3N TWO

Ideally a naming system should tell you everything about a structure without ambiguity. There are two types of naming system commonly found in organic chemistry;

Trivial : based on some property or historical aspect;
the name tells you little about the structure
Systematic : based on an agreed set of rules (I.U.P.A.C);
exact structure can be found from the name (and vice-versa).

HOMOLOGOUS SERIES
trivial name systematic name example(s)
paraffin alkane methane, butane
olefin alkene ethene, butene
fatty acid alkanoic (carboxylic) acid ethanoic acid


INDIVIDUAL COMPOUNDS
trivial name derivation systematic name
methane methu = wine (Gk.) methane (CH4)
butane butyrum = butter (Lat.) butane (C4H10)
acetic acid acetum = vinegar (Lat.) ethanoic acid (CH3COOH)

A systematic name has two main parts.

STEM number of carbon atoms in longest chain bearing the functional group +
a prefix showing the position and identity of any side-chain substituents.

Apart from the first four, which have trivial names, the number of carbons atoms is indicated by a prefix derived from the Greek numbering system.


The list of alkanes demonstrate the use of prefixes.

The ending -ane is the same as they are all alkanes.

How long is a chain?

Because organic molecules are three dimensional and paper is two dimensional it can confusing when comparing molecules. This is because...

1. It is too complicated to draw molecules with the correct bond angles
2. Single covalent bonds are free to rotate

All the following written structures are of the same molecule - PENTANE C5H12

How long is the longest chain?

Look at the structures and work out how many carbon atoms are in the longest chain.

How long is the longest chain?

Look at the structures and work out how many carbon atoms are in the longest chain.

SUBSTITUENTS Many compounds have substituents (additional atoms, or groups)
attached to the chain. Their position is numbered.

A systematic name has two main parts.

SUFFIX An ending that tells you which functional group is present

See if any functional groups are present.
Add relevant ending to the basic stem.

In many cases the position of the functional group must be given to avoid any ambiguity

SIDE-CHAIN carbon based substituents are named before the chain name.
they have the prefix -yl added to the basic stem (e.g. CH3 is methyl).







Number the principal chain from one end to give the lowest numbers.
Side-chain names appear in alphabetical order butyl, ethyl, methyl, propyl
Each side-chain is given its own number.
If identical side-chains appear more than once, prefix with di, tri, tetra, penta, hexa
Numbers are separated from names by a HYPHEN e.g. 2-methylheptane
Numbers are separated from numbers by a COMMA e.g. 2,3-dimethylbutane

SIDE-CHAIN carbon based substituents are named before the chain name.
they have the prefix -yl added to the basic stem (e.g. CH3 is methyl).







Number the principal chain from one end to give the lowest numbers.
Side-chain names appear in alphabetical order butyl, ethyl, methyl, propyl
Each side-chain is given its own number.
If identical side-chains appear more than once, prefix with di, tri, tetra, penta, hexa
Numbers are separated from names by a HYPHEN e.g. 2-methylheptane
Numbers are separated from numbers by a COMMA e.g. 2,3-dimethylbutane


Example longest chain 8 (it is an octane)
3,4,6 are the numbers NOT 3,5,6
order is ethyl, methyl, propyl

3-ethyl-5-methyl-4-propyloctane

Length In alkenes the principal chain is not always the longest chain
It must contain the double bond
the name ends in -ENE

Position Count from one end as with alkanes.
Indicated by the lower numbered carbon atom on one end of the C=C bond


5 4 3 2 1
CH3CH2CH=CHCH3 is pent-2-ene (NOT pent-3-ene)


Side-chain Similar to alkanes
position is based on the number allocated to the double bond

1 2 3 4 1 2 3 4
CH2 = CH(CH3)CH2CH3 CH2 = CHCH(CH3)CH3
2-methylbut-1-ene 3-methylbut-1-ene

Elucidation of the structures of organic compounds - a brief summary

Organic chemistry is so vast that the identification of a compound can be involved. The characterisation takes place in a series of stages (see below). Relatively large amounts of substance were required to elucidate the structure but, with modern technology and the use of electronic instrumentation, very small amounts are now required.


Elemental composition
One assumes that organic compounds contain carbon and hydrogen but it can be proved by letting the compound undergo combustion. Carbon is converted to carbon dioxide and hydrogen is converted to water.

Percentage composition by mass
Found by dividing the mass of an element present by the mass of the compound present, then multiplying by 100. Elemental mass of C and H can be found by allowing the substance to undergo complete combustion. From this one can find...

mass of carbon = 12/44 of the mass of CO2 produced
mass of hydrogen = 2/18 of the mass of H2O produced

Chemical Chemical reactions can identify the functional group(s) present.


Spectroscopy IR detects bond types due to absorbance of i.r. radiation

NMR gives information about the position and relative
numbers of hydrogen atoms present in a molecule



Confirmation By comparison of IR or NMR spectra and
mass spectrometry

What should you be able to do?

Recall and explain the reasons for the large number of carbon based compounds
Be able to write out possible structures for a given molecular formula
Recognize the presence of a particular functional group in a structure
Know the IUPAC rules for naming alkanes and alkenes
Be able to name given alkanes and alkenes when given the structure
Be able to write out the structure of an alkane or alkene when given its name
Recall the methods used to characterise organic molecules

CAN YOU DO ALL OF THESE? YES NO

AN INTRODUCTION TO ORGANIC CHEMISTRY

THE END