Fuels and Heats of Reaction

Chapter 21

Fuels and Heats of Reaction

• Organic chemistry is defined as the study of the compounds of carbon.

• A hydrocarbon is a compounds of carbon and hydrogen only.

Types of hydrocarbon.• Aliphatic- have molecules in chains or rings of carbon

atoms.• Aromatic - hydrocarbons with a Benzene ring of 6 carbon

atoms, where the bonds are intermediate between single and double bonds.

Organic chemistry

Aliphatic hydrocarbons.

• A homologous series is a group of compounds with similar chemical properties due to having the same functional group. They have a gradual variation in physical properties and they

differ from each other by CH2.

• A functional group is a group of atoms within the molecule which, due to its structure, gives the molecule its characteristics.(-OH functional group gives alcohols their characteristics)

• The 3 main classes of Hydrocarbons are Alkanes, Alkenes and Alkynes.

Alkanes• Formerly known as the ‘paraffins’ (from the Latin meaning

‘lacking affinity’) the alkanes are a group of hydrocarbons which conform to the general formula CnH2n+2, where n is greater than or equal to 1. They are saturated compounds. They consist of carbon atoms joined by four single covalent bonds to either hydrogen or other carbon atoms.

• The first member of the group is methane (CH4), the second is ethane (C2H6), and the third is propane (C3H8), as illustrated in Fig. 3.2. By drawing their structural formulae we can see that each differs from the next by replacing a hydrogen with a CH3 group, i.e. an extra CH2 is inserted in the chain.

General members of a homologous seriesgeneral formula is CnH2n+2 - for non-cyclic alkanes

saturated hydrocarbons - all carbon-carbon bonding is single bonds are spaced tetrahedrally about carbon atoms.

Isomerism the first example of structural isomerism occurs with C4H10

BUTANE 2-METHYLPROPANE

Structural isomers have the SAME MOLECULAR FORMULA BUTDIFFERENT STRUCTURAL FORMULA

They possess different physical properties such as boiling point,melting point and density

ALKANESALKANES

In ALKANES, the four sp3 orbitals of carbon repel each other into a TETRAHEDRAL arrangement with bond angles of 109.5º.

Each sp3 orbital in carbon overlaps with the 1s orbital of a hydrogen atom to form a C-H bond.

THE STRUCTURE OF ALKANESTHE STRUCTURE OF ALKANES

109.5º

Nomenclature of alkanesNaming straight-chain alkanes• Naming of straight chain alkanes (alkanes that do not branch) is a

straightforward process. To give an alkane a name, a prefix indicating the number of carbons in the molecule is added to the suffix ane, identifying both the kind of molecule (an alkane) and how many carbons the molecule has (the prefix). The name pentane, for example, tells you that the molecule is an alkane (thus the ane ending) and that it has five carbons (pent indicates five). Prefixes for alkanes that have 1-4 carbons are rooted historically. These are methane, ethane, propane, and butane, respectively. On the other hand, for 5 carbons and up a prefix derived from greek is given. (An easy way to remember the first four names is the anagram, standing for methane, ethane, propane, butane). Learning the prefixes for up to twelve carbons is a good idea, and they are listed in the figure below.

Molecular Formula Structural Formula IUPAC Name

CH4 CH4 Methane

C2H6 CH3-CH3 Ethane

C3H8 CH3-CH2-CH3 Propane

C4H10 CH3-(CH2)2-CH3 Butane

C5H12 CH3-(CH2)3-CH3 Pentane

C6H14 CH3-(CH2)4-CH3 Hexane

C7H16 CH3-(CH2)5-CH3 Heptane

C8H18 CH3-(CH2)6-CH3 Octane

C9H20 CH3-(CH2)7-CH3 Nonane

C10H22 CH3-(CH2)8-CH3 Decane

Naming branched alkanes• There are several rules that you must follow to give the

alkane the correct name.

1. Find the longest chain of carbons in the molecule. The number of carbons in the longest chain becomes the parent name (refer to the above table)

2. After finding the parent chain, you number the parent chain starting with the end nearest the first substituent (a substituent is any fragment that juts off the main chain).

3. Next, determine the names of all substituents. Substituents are named as if the piece were a separate molecule, except that the suffix of yl is used rather than ane. Thus, a two-carbon substituent would be an ethyl substituent (not an ethane substituent).

4. Put the substituents in alphabetical order (ie. ethyl before methyl) in front of the parent name.

5. Next, identify the positions of all substituents in the name by placing the carbon number where the substituent attaches to the parent chain in front of it. For example, 2-methylheptane indicates that a methyl substituent is attached to the number 2 carbon

Mary eats peanut butter

Boiling point increases as they get more carbon atoms in their formula more atoms = greater intermolecular Van der Waals’ forces greater intermolecular force = more energy to separate the molecules greater energy required = higher boiling point

CH4 (-161°C) C2H6 (-88°C) C3H8 (-42°C) C4H10 (-0.5°C)

difference gets less - mass increases by a smaller percentage straight chains molecules have greater interaction than branched

“The greater the branching, the lower the boiling point”

Melting point general increase with molecular massthe trend is not as regular as that for boiling point.

Solubilityalkanes are non-polar so are immiscible with waterthey are soluble in most organic solvents.

PHYSICAL PROPERTIES OF ALKANESPHYSICAL PROPERTIES OF ALKANES

Introduction - fairly unreactive; (old family name, paraffin, meant little reactivity) - have relatively strong, almost NON-POLAR, SINGLE covalent bonds - they have no real sites that will encourage substances to attack them

Combustion - make useful fuels - especially the lower members of the series - react with oxygen in an exothermic reaction

complete CH4(g) + 2O2(g) ——> CO2(g) + 2H2O(l)combustion

incomplete CH4(g) + 1½O2(g) ——> CO(g) + 2H2O(l)combustion

the greater the number of carbon atoms, the more energy produced BUT the greater the amount of oxygen needed for complete combustion.

Handy tip When balancing equations involving complete combustion, remember...every carbon in the original hydrocarbon gives one carbon dioxide andevery two hydrogen atoms gives a water molecule.

Put the numbers into the equation, count up the O’s and H’s on the RHSof the equation then balance the oxygen molecules on the LHS.

CHEMICAL PROPERTIES OF ALKANESCHEMICAL PROPERTIES OF ALKANES

Processes involving combustion give rise to a variety of pollutants...

power stations SO2 emissions produce acid raininternal combustion engines CO, NOx and unburnt hydrocarbons

RemovalSO2 react effluent gases with a suitable compound (e.g. CaO)CO and NOx pass exhaust gases through a catalytic converter

Catalytic convertersIn the catalytic converter ... CO is converted to CO2

NOx are converted to N2

Unburnt hydrocarbons are converted to CO2 and H2O

e.g. 2NO + 2CO ———> N2 + 2CO2

• catalysts are made of finely divided rare metals Rh, Pd, Pt• leaded petrol must not pass through the catalyst as the lead deposits on the catalyst’s surface and “poisons” it, thus blocking sites for reactions to take place.

POLLUTIONPOLLUTION

Alkenes

Alkenes General Formula CnH2n

• Ethene C2H4

• Propene C3H6

• But-1-ene C4H8

• But-2-ene C4H8

Alkenes

• Alkenes are unsaturated hydrocarbons, as they contain a carbon-carbon double bond

Ethene C2H4

Propene C3H6

But-1-ene C4H8

But-2-ene C4H8

Alkenes

• Alkene molecules are planar, e.g. ethene

Physical properties

• Physical state: Gases• Insoluble in water• Soluble in non-polar solvents such as

cyclohexane

Uses

• Used to make plastics, e.g. ethene is used to make polythene

• Ethene is used to make ethanol for industrial use

Involves the breaking of C-C bonds in alkanes Converts heavy fractions into higher value products

THERMAL proceeds via a free radical mechanismCATALYTIC proceeds via a carbocation (carbonium ion) mechanism

CRACKINGCRACKING

CATALYTIC

SLIGHT PRESSURE HIGH TEMPERATURE ... 450°C ZEOLITE CATALYST CARBOCATION (IONIC) MECHANISM HETEROLYTIC FISSION PRODUCES BRANCHED AND CYCLIC ALKANES, AROMATIC HYDROCARBONS USED FOR MOTOR FUELS

ZEOLITES are crystalline aluminosilicates; clay like substances

Involves the breaking of C-C bonds in alkanes Converts heavy fractions into higher value products

THERMAL proceeds via a free radical mechanismCATALYTIC proceeds via a carbocation (carbonium ion) mechanism

CRACKINGCRACKING

THERMAL

HIGH PRESSURE ... 7000 kPa HIGH TEMPERATURE ... 400°C to 900°C FREE RADICAL MECHANISM HOMOLYTIC FISSION PRODUCES MOSTLY ALKENES ... e.g. ETHENE for making polymers and ethanol PRODUCES HYDROGEN ... used in the Haber Process and in margarine manufacture

Bonds can be broken anywhere in the molecule by C-C bond fission or C-H bond fission

Petrol

Petrol is formed by the fractional distillation of crude oil.

Some of the different fractions formed are: Petroleum Gas - Mercaptans added for smell.Petrol- light gasolineNaphta - Petrochemical IndustryKerosene - Aviation Fuel.Diesel.Lubricating Oil.•

Octane NumberKnocking - early explosion due to petrol-air mixture. The petrol explodes

as it is compressed instead of being caused by a spark. (Auto-ignition). Octane Number - Measure of the tendency of fuels to resist knocking. 2, 2-4 tri-methyl pentane (iso-octane) has an octane number of 100.Heptane is not efficient and has an octane number of 0. The shorter the alkane the higher the octane number.The more branched the chain the higher the octane number.Cyclic compounds have a higher octane number than straight chain

compounds.

Making PetrolKnocking - early explosion due to petrol-air mixture. The petrol explodes as it is

compressed instead of being caused by a spark. (Auto-ignition). Tetra-ethyl lead -Reduces knocking. However it caused health problems and is poisoness. 4 ways to increase octane number. 1. Isomerisation. Branching caused by alkanes heated in the presence of a suitable

catalyst.2. Catalytic Cracking. Long chained hydrocarbons broken down into short chain

molecules for which there is great demand. Large alkanes form alkanes and alkenes.3. Reforming (DeHydrocyclisation.) Reforming involves the use of catalysts to form ring

compounds.-Hydrogen given off.4. Adding Oxygenates. Three oxygen containing molecules, methanol, ethanol, and

MTBE are added to petrol to increase its octane number. MTBE stands for methyl tertiary butyl ether. (2-methoxy-2-methylpropane)

CONTENTS

INTRODUCTION

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KNOCKHARDY PUBLISHINGKNOCKHARDY PUBLISHING

ORGANIC CHEMISTRYORGANIC CHEMISTRY

CONTENTS

ORGANIC CHEMISTRYORGANIC 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 CHAINSBRANCHED 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

CONTENTS

SPECIAL NATURE OF CARBON - SPECIAL NATURE OF CARBON - CATENATIONCATENATION

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 RINGSCARBON ATOMS CAN BE ARRANGED IN

STRAIGHT CHAINS

BRANCHED CHAINS

and RINGS

CONTENTS

THE SPECIAL NATURE OF CARBONTHE SPECIAL NATURE OF CARBON

You can also get a combination of rings and chains

MULTIPLE BONDING AND SUBSTITUENTS

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

CONTENTS



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 SULPHURSULPHUR are very common.

CARBON SKELETON FUNCTIONAL CARBON SKELETON FUNCTIONAL GROUP GROUP

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

CONTENTS



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

CONTENTS


THE C=C DOUBLE BOND IS IN A DIFFERENT POSITION

THE CHLORINE ATOM IS IN A DIFFERENT POSITION

PENT-1-ENE PENT-2-ENE

1-CHLOROBUTANE 2-CHLOROBUTANE

CONTENTS

TYPES OF FORMULAE - 1TYPES OF FORMULAE - 1

MOLECULAR FORMULA C4H10

The exact number of atoms of eachelement present in the molecule

EMPIRICAL FORMULA C2H5

The simplest whole number ratioof atoms in the molecule

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

DISPLAYED FORMULAShows both the relative placing of atomsand the number of bonds between them

H C C C C H

H H H H

H H H H

H C C C HH H

H H H

H C HH

THE EXAMPLE BEING USED IS... BUTANE

SKELETAL FORMULAA 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

CONTENTS


CH2 CH2

CH2 CH2

CH2

CH2

CYCLOHEXANE THALIDOMIDE

for

SKELETAL FORMULAA 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 FORMULARepresents 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

CONTENTS


CH2 CH2

CH2 CH2

CH2

CH2

CYCLOHEXANE THALIDOMIDE

for

CONTENTS

HOMOLOGOUS SERIESHOMOLOGOUS 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 C2H5OH C3H7OHMETHANOL ETHANOL PROPAN-1-OL

CONTENTS

FUNCTIONAL GROUPSFUNCTIONAL 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 C C C C C OH

H H H H H

H H H H H

H C C C C C NH2

H H H H HCarbon Functional Carbon Functionalskeleton Group = AMINE skeleton Group = ALCOHOL

CONTENTS

COMMON FUNCTIONAL GROUPSCOMMON FUNCTIONAL GROUPS

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

CONTENTS

COMMON FUNCTIONAL GROUPSCOMMON FUNCTIONAL GROUPS

ALKANE

ALKENE

ALKYNE

HALOALKANE

AMINE

NITRILE

ALCOHOL

ETHER

ALDEHYDE

KETONE

CARBOXYLIC ACID

ESTER

ACYL CHLORIDE

AMIDE

NITRO

SULPHONIC ACID

CONTENTS

HOW MANY STRUCTURES?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 4 covalent bonds surrounding themoxygen atoms 2nitrogen atoms 3hydrogen 1halogen atoms 1

C2H6 ONE

C3H9Br 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

CONTENTS

NOMENCLATURENOMENCLATURE

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 SERIEStrivial name systematic name example(s)paraffin alkane methane, butaneolefin alkene ethene, butenefatty acid alkanoic (carboxylic) acid ethanoic acid

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

CONTENTS

I.U.P.A.C. NOMENCLATUREI.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 C atoms Alkanemeth- 1 methaneeth- 2 ethaneprop- 3 propanebut- 4 butanepent- 5 pentanehex- 6 hexanehept- 7 heptaneoct- 8 octanenon- 9 nonanedec- 10 decane

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

CH2CH3 CH2 CH2 CH3 CH2 CH2CH2

CH3

CH3

CH3

CH2 CH2CH2

CH3

CH2CH2

CH2CH3 CH3


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 angles2. Single covalent bonds are free to rotate

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

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.

CONTENTS

CH2CH3 CH2 CH2 CH CH3

CH3

CH2CH3 CH3CHCH2

CH3


How long is the longest chain?

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

CH3

CH3CHCH2

CH2CH3 CHCH3

THE ANSWERS AREON THE NEXT SLIDE

CONTENTS


CH3

CH2CH3 CH3CHCH2

CH3


How long is the longest chain?

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

CH3

CH3CHCH2

CH2CH3 CHCH3

LONGEST CHAIN = 5

LONGEST CHAIN = 6

LONGEST CHAIN = 6

CONTENTS

CONTENTS


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

Functional group SuffixALKANE - ANEALKENE - ENEALKYNE - YNEALCOHOL - OLALDEHYDE - ALKETONE - ONEACID - OIC ACID

1-CHLOROBUTANE 2-CHLOROBUTANE

CONTENTS

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, propylEach side-chain is given its own number.

If identical side-chains appear more than once, prefix with di, tri, tetra, penta, hexaNumbers are separated from names by a HYPHEN e.g. 2-methylheptaneNumbers are separated from numbers by a COMMA e.g. 2,3-dimethylbutane

Alkyl radicals methyl CH3 - CH3

ethyl CH3- CH2- C2H5

propyl CH3- CH2- CH2- C3H7


CONTENTS

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, propylEach side-chain is given its own number.

If identical side-chains appear more than once, prefix with di, tri, tetra, penta, hexaNumbers are separated from names by a HYPHEN e.g. 2-methylheptaneNumbers 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 3,5,6order is ethyl, methyl, propyl

3-ethyl-5-methyl-4-propyloctane

Alkyl radicals methyl CH3 - CH3

ethyl CH3- CH2- C2H5

propyl CH3- CH2- CH2- C3H7

CH3

CH2 CH3CHCH2

CH2CH3 CHCH

CH2

CH2CH3 CH2

CH3



CH3

CH2CH3 CH3CHCH2

CH3


Apply the rules and name these alkanes

CH3

CH3CHCH2

CH2CH3 CHCH3

THE ANSWERS ARE ON THE NEXT SLIDE

CONTENTS


CH3

CH2CH3 CH3CHCH2

CH3


CH3

CH3CHCH2

CH2CH3 CHCH3



CONTENTS


CH3

CH2CH3 CH3CHCH2

CH3


CH3

CH3CHCH2

CH2CH3 CHCH3

Longest chain = 5 so it is a pentane

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

3-methylpentane



CONTENTS


CH3

CH2CH3 CH3CHCH2

CH3


CH3

CH3CHCH2

CH2CH3 CHCH3



3-methylpentane



Longest chain = 6 so it is a hexane

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

2-methylhexane

CONTENTS


CH3

CH2CH3 CH3CHCH2

CH3


CH3

CH3CHCH2

CH2CH3 CHCH3



3-methylpentane




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

2-methylhexane


CH3, methyl, groups are attached to the third and fourth carbon atoms (whichever end you count from).

3,4-dimethylhexane

CONTENTS

CONTENTS

NAMING ALKENESNAMING 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 1CH3CH2CH=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

CONTENTS

WHICH COMPOUND IS IT?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 compositionOne 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 massFound 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

CONTENTS

INVESTIGATING MOLECULESINVESTIGATING MOLECULES

Empirical formulaThe 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.

CONTENTS



Molecular massTraditionally 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.

CONTENTS




Molecular formulaThe 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 78the molecular formula will be 78/13 or 6 times the empirical formula i.e. C6H6 .

CONTENTS




Molecular formulaThe 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 78the molecular formula will be 78/13 or 6 times the empirical formula i.e. C6H6 .

Structural formulaBecause 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.

CONTENTS


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 relativenumbers of hydrogen atoms present in a molecule

Confirmation By comparison of IR or NMR spectra and mass spectrometry

REVISION CHECKREVISION CHECKWhat 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? CAN YOU DO ALL OF THESE? YES YES NONO

CONTENTS

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© 2003 JONATHAN HOPTON & KNOCKHARDY PUBLISHING© 2003 JONATHAN HOPTON & KNOCKHARDY PUBLISHING

AN INTRODUCTION TOAN INTRODUCTION TO ORGANIC ORGANIC CHEMISTRYCHEMISTRY

THE ENDTHE END

Fuels and Heats of Reaction

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