Organic Chemistry A Levels A2

AN INTRODUCTION TO

ORGANIC CHEMISTRY A guide for A level students

KNOCKHARDY PUBLISHING

2008 SPECIFICATIONS

KNOCKHARDY PUBLISHING

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

ORGANIC CHEMISTRY Before you start it would be helpful to… • Recall how covalent bonding arises • Recall simple electron pair repulsion theory

ORGANIC CHEMISTRY 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

SPECIAL NATURE OF CARBON - CATENATION 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.

THE SPECIAL NATURE OF CARBON

CHAINS AND RINGS CARBON ATOMS CAN BE ARRANGED IN

STRAIGHT CHAINS

BRANCHED CHAINS

and

RINGS

You can also get a combination of rings and chains

THE SPECIAL NATURE OF CARBON

MULTIPLE BONDING AND SUBSTITUENTS CARBON-CARBON COVALENT BONDS CAN BE SINGLE, DOUBLE OR TRIPLE

THE SPECIAL NATURE OF CARBON

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 GROUP

CARBON SKELETON

FUNCTIONAL GROUP

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

THE SPECIAL NATURE OF CARBON

MULTIPLE BONDING AND SUBSTITUENTS ATOMS/GROUPS CAN BE PLACED IN DIFFERENT POSITIONS ON A CARBON SKELETON THE C=C DOUBLE BOND IS IN A DIFFERENT POSITION

PENT-1-ENE

PENT-2-ENE

THE CHLORINE ATOM IS IN A DIFFERENT POSITION

1-CHLOROBUTANE

2-CHLOROBUTANE

TYPES OF FORMULAE - 1 MOLECULAR FORMULA The exact number of atoms of each element present in the molecule

C4H10

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

C2H5

STRUCTURAL FORMULA The minimal detail using conventional groups, for an unambiguous structure

CH3CH2CH2CH3

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

THE EXAMPLE BEING USED IS...

BUTANE

CH3CH(CH3)CH3

there are two possible structures

H

H

H

H

H

C

C

C

C

H

H

H

H

H

H

H

H

H

C

C

C

H H H C H H

H

TYPES OF FORMULAE - 2 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 CH2 CH2

CH2

CH2

CH2

for CH2

CYCLOHEXANE

THALIDOMIDE

TYPES OF FORMULAE - 2 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 CH2 CH2

CH2

CH2

CH2

for CH2

CYCLOHEXANE

GENERAL FORMULA Represents any member of a homologous series

THALIDOMIDE

for alkanes it is... possible formulae...

CnH2n+2 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

HOMOLOGOUS SERIES 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.

ALCOHOLS - FIRST THREE MEMBERS OF THE SERIES

CH3OH METHANOL

C2H5OH ETHANOL

C3H7OH PROPAN-1-OL

FUNCTIONAL GROUPS Organic chemistry is a vast subject so it is easier to split it into small sections for study. This is done by studying compounds which behave in a similar way because they have a particular atom, or group of atoms, FUNCTIONAL GROUP, in their structure. Functional groups can consist of one atom, a group of atoms or multiple bonds between carbon atoms. Each functional group has its own distinctive properties which means that the properties of a compound are governed by the functional group(s) in it.

H H H H H

H H H H H H C C C C C

NH2

OH

H H H H H

H H H H H Carbon skeleton

H C C C C C

Functional Group = AMINE

Carbon skeleton

Functional Group = ALCOHOL

COMMON FUNCTIONAL GROUPS GROUP

ENDING

ALKANE

- ane

ALKENE ALKYNE

GENERAL FORMULA RH

EXAMPLE C2H6

ethane

- ene

C2H4

ethene

- yne

C2H2

ethyne

HALOALKANE

halo -

RX

C2H5Cl

chloroethane

ALCOHOL

- ol

ROH

C2H5OH

ethanol

-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

ALDEHYDE

SULPHONIC ACID

- sulphonic acid

RSO3H

C6H5SO3H

benzene sulphonic acid

ETHER

- oxy - ane

ROR

C2H5OC2H5

ethoxyethane

COMMON FUNCTIONAL GROUPS ALKANE

CARBOXYLIC ACID

ALKENE ALKYNE

ESTER

HALOALKANE AMINE NITRILE

ACYL CHLORIDE

AMIDE

ALCOHOL ETHER

NITRO

ALDEHYDE

KETONE

SULPHONIC ACID

HOW MANY STRUCTURES? 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 oxygen atoms nitrogen atoms hydrogen halogen atoms

4 covalent bonds surrounding them 2 3 1 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

HOW MANY STRUCTURES? 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 oxygen atoms nitrogen atoms hydrogen halogen atoms

4 covalent bonds surrounding them 2 3 1 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

NOMENCLATURE 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 : Systematic :

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

trivial name paraffin olefin fatty acid

trivial name methane butane acetic acid

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

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

I.U.P.A.C. NOMENCLATURE 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.

Prefix methethpropbutpenthexheptoctnondec-

C atoms 1 2 3 4 5 6 7 8 9 10

Alkane methane ethane propane butane pentane hexane heptane octane nonane decane

Working out which is the longest chain can pose a problem with larger molecules.

I.U.P.A.C. NOMENCLATURE 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

CH3 CH2 CH2 CH2 CH3

CH3

CH2 CH2 CH2 CH3

CH3 CH2 CH2 CH2

CH3 CH2 CH3

CH3

CH2 CH2

A simple way to check is to run a finger along the chain and see how many carbon atoms can be covered without reversing direction or taking the finger off the page. In all the above there are... FIVE CARBON ATOMS IN A LINE.

I.U.P.A.C. NOMENCLATURE How long is the longest chain? Look at the structures and work out how many carbon atoms are in the longest chain.

CH3

THE ANSWERS ARE ON THE NEXT SLIDE

CH2 CH3 CH CH2 CH3 CH3 CH3 CH2 CH2 CH2 CH CH3 CH3 CH3 CH2 CH3 CH2 CH CH CH3

I.U.P.A.C. NOMENCLATURE How long is the longest chain? Look at the structures and work out how many carbon atoms are in the longest chain.

CH3 LONGEST CHAIN = 5

CH2 CH3 CH CH2 CH3 CH3

LONGEST CHAIN = 6

CH3 CH2 CH2 CH2 CH CH3 CH3 CH3 CH2 CH3 CH2 CH CH CH3

LONGEST CHAIN = 6

I.U.P.A.C. NOMENCLATURE 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

1-CHLOROBUTANE

SUBSTITUENTS

Functional group

Suffix

ALKANE ALKENE ALKYNE ALCOHOL ALDEHYDE KETONE ACID

- ANE - ENE - YNE - OL - AL - ONE - OIC ACID

2-CHLOROBUTANE

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

I.U.P.A.C. NOMENCLATURE 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). Alkyl radicals

methyl

CH3 -

CH3

ethyl propyl

CH3- CH2CH3- CH2- CH2-

C2H5 C3H7

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

I.U.P.A.C. NOMENCLATURE 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). Alkyl radicals

methyl

CH3 -

CH3

ethyl propyl

CH3- CH2CH3- CH2- CH2-

C2H5 C3H7

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.

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

2-methylheptane

CH3 CH3 CH3 CH2 CH2 CH

CH2

CH3 CH2 CH2 CH

CH

CH2

CH3

I.U.P.A.C. NOMENCLATURE Apply the rules and name these alkanes THE ANSWERS ARE ON THE NEXT SLIDE

CH3 CH2 CH3 CH CH2 CH3 CH3 CH3 CH2 CH2 CH2 CH CH3

CH3 CH3 CH2

CH3 CH2 CH CH CH3

I.U.P.A.C. NOMENCLATURE Apply the rules and name these alkanes

CH3 CH2 CH3 CH CH2 CH3

CH3 CH3 CH2 CH2 CH2 CH CH3

CH3 CH3 CH2 CH3 CH2 CH CH CH3

I.U.P.A.C. NOMENCLATURE Apply the rules and name these alkanes Longest chain = 5 so it is a pentane

CH3

A CH3, methyl, group is attached to the third carbon from one end...

CH2 CH3 CH CH2 CH3

3-methylpentane

CH3 CH3 CH2 CH2 CH2 CH CH3

CH3 CH3 CH2 CH3 CH2 CH CH CH3

I.U.P.A.C. NOMENCLATURE Apply the rules and name these alkanes Longest chain = 5 so it is a pentane

CH3

A CH3, methyl, group is attached to the third carbon from one end...

CH2 CH3 CH CH2 CH3

3-methylpentane

CH3 CH3 CH2 CH2 CH2 CH CH3

Longest chain = 6 so it is a hexane A CH3, methyl, group is attached to the second carbon from one end... 2-methylhexane

CH3 CH3 CH2 CH3 CH2 CH CH CH3

I.U.P.A.C. NOMENCLATURE Apply the rules and name these alkanes Longest chain = 5 so it is a pentane

CH3

A CH3, methyl, group is attached to the third carbon from one end...

CH2 CH3 CH CH2 CH3

3-methylpentane

CH3 CH3 CH2 CH2 CH2 CH CH3

Longest chain = 6 so it is a hexane A CH3, methyl, group is attached to the second carbon from one end... 2-methylhexane

CH3 CH3 CH2 CH3 CH2 CH CH CH3

Longest chain = 6 so it is a hexane CH3, methyl, groups are attached to the third and fourth carbon atoms (whichever end you count from). 3,4-dimethylhexane

NAMING ALKENES 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 Side-chain

is pent-2-ene

(NOT pent-3-ene)

Similar to alkanes position is based on the number allocated to the double bond 1

2

3

4

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

1

2

3

4

CH2 = CHCH(CH3)CH3 3-methylbut-1-ene

WHICH COMPOUND IS IT? 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 mass of hydrogen

= =

12/44 of the mass of CO2 produced 2/18 of the mass of H2O produced

INVESTIGATING MOLECULES Empirical formula The simplest ratio of elements present in the substance. It is calculated by dividing the mass or percentage mass of each element by its molar mass and finding the simplest ratio between the answers. Empirical formula is converted to the molecular formula using molecular mass.

INVESTIGATING MOLECULES Empirical formula The simplest ratio of elements present in the substance. It is calculated by dividing the mass or percentage mass of each element by its molar mass and finding the simplest ratio between the answers. Empirical formula is converted to the molecular formula using molecular mass. Molecular mass Traditionally found out using a variety of techniques such as ... volumetric analysis or molar volume methods (Dumas, Victor-Meyer or gas syringe experiments). Mass spectrometry is now used. The m/z value of the molecular ion and gives the molecular mass. The fragmentation pattern gives information about the compound.

INVESTIGATING MOLECULES Empirical formula The simplest ratio of elements present in the substance. It is calculated by dividing the mass or percentage mass of each element by its molar mass and finding the simplest ratio between the answers. Empirical formula is converted to the molecular formula using molecular mass. Molecular mass Traditionally found out using a variety of techniques such as ... volumetric analysis or molar volume methods (Dumas, Victor-Meyer or gas syringe experiments). Mass spectrometry is now used. The m/z value of the molecular ion and gives the molecular mass. The fragmentation pattern gives information about the compound. Molecular formula The molecular formula is an exact multiple of the empirical formula. Comparing the molecular mass with the empirical mass allows one to find the true formula. e.g. if the empirical formula is CH (relative mass = 13) and the molecular mass is 78 the molecular formula will be 78/13 or 6 times the empirical formula i.e. C6H6 .

INVESTIGATING MOLECULES Empirical formula The simplest ratio of elements present in the substance. It is calculated by dividing the mass or percentage mass of each element by its molar mass and finding the simplest ratio between the answers. Empirical formula is converted to the molecular formula using molecular mass. Molecular mass Traditionally found out using a variety of techniques such as ... volumetric analysis or molar volume methods (Dumas, Victor-Meyer or gas syringe experiments). Mass spectrometry is now used. The m/z value of the molecular ion and gives the molecular mass. The fragmentation pattern gives information about the compound. Molecular formula The molecular formula is an exact multiple of the empirical formula. Comparing the molecular mass with the empirical mass allows one to find the true formula. e.g. if the empirical formula is CH (relative mass = 13) and the molecular mass is 78 the molecular formula will be 78/13 or 6 times the empirical formula i.e. C6H6 . Structural formula Because of the complexity of organic molecules, there can be more than one structure for a given molecular formula. To work out the structure, different tests are carried out.

INVESTIGATING MOLECULES Empirical formula The simplest ratio of elements present in the substance. It is calculated by dividing the mass or percentage mass of each element by its molar mass and finding the simplest ratio between the answers. Empirical formula is converted to the molecular formula using molecular mass. Molecular mass Traditionally found out using a variety of techniques such as ... volumetric analysis or molar volume methods (Dumas, Victor-Meyer or gas syringe experiments). Mass spectrometry is now used. The m/z value of the molecular ion and gives the molecular mass. The fragmentation pattern gives information about the compound. Molecular formula The molecular formula is an exact multiple of the empirical formula. Comparing the molecular mass with the empirical mass allows one to find the true formula. e.g. if the empirical formula is CH (relative mass = 13) and the molecular mass is 78 the molecular formula will be 78/13 or 6 times the empirical formula i.e. C6H6 . Structural formula Because of the complexity of organic molecules, there can be more than one structure for a given molecular formula. To work out the structure, different tests are carried out.

INVESTIGATING MOLECULES 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

By

comparison of IR or NMR spectra and mass spectrometry

Confirmation

REVISION CHECK

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

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AN INTRODUCTION TO

ORGANIC CHEMISTRY THE END

© 2008 JONATHAN HOPTON & KNOCKHARDY PUBLISHING