Chemical Ideas 12 Organic chemistry frameworks 12.1 Alkanes

Many carbon compounds are found in living organisms, this why their study is named organic chemistry

Carbon is unique About 7 million compounds containing carbon and hydrogen are known to chemists This is far more than the number of compounds from all the other elements put together

Why carbon? Electron structure makes it the first member of Gp 4 in the centre of the periodic table – this is responsible for its special properties Carbon atom has 4 electrons in its outer shell – too many to lose or gain – the ions would have +4 or -4 (too highly charged) All carbon compounds are covalent rather than ionic Methane(CH 4 ) C shares 4 electrons with 4 H atoms Carbon forms strong covalent bonds with itself to form rings and chains this is called catenation Each C atom can form 4 covalent bonds- chains may be straight or branched and can have other atoms or groups substituted on to them H  H  C  H   H

Hydrocarbons Only contain carbon and hydrogen General formula C X H Y Methane –CH 4 is an alkane Ethene C 2 H 4 is an alkene Benzene C 6 H 6 is an arene H l H - C - H l H

Alkanes Saturated hydrocarbons Contain C and H only Contain single bonds C-C Have 4 bonds to every carbon (C) atom Are non polar General formula C n H 2n+2 Physical properties such as m.pt,b.pt and density change as the number of carbon atoms in the molecule increase

Alkanes shortened Name# carbons Structural Formula Methane1CH 4 Ethane2CH 3 CH 3 Propane3CH 3 CH 2 CH 3 Butane4CH 3 CH 2 CH 2 CH 3 Pentane5CH 3 CH 2 CH 2 CH 2 CH 3 Names of all alkanes end in -ane

Alkanes shortened Name # carbons Structural Formula Hexane 6 CH 3 CH 2 CH 2 CH 2 CH 2 CH 3 Heptane 7 CH 3 CH 2 CH 2 CH 2 CH 2 CH 2 CH 3 Octane 8 CH 3 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 CH 3 Nonane 9 CH 3 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 CH 3 Decane 10 CH 3 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 CH 3 Names of all alkanes end in -ane A series of compounds which are related in this way are called a homologous series

Finding the formula of alkanes Molecular formula = m x empirical formula Where m is 1,2,3,…… For hydrocarbons, composition of mass is easily found by burning a known mass in oxygen, measuring the amounts of CO 2 and H 2 O this is called combustion analysis

Finding the formula of alkanes Example 0.100g of hydrocarbon X on complete combustion gave 0.309g CO 2 and H 2 O 0.142g calculate the empirical formula of X Answer 1 st calc the masses of C and H in 0.100g 44g CO 2 contains 12g of C mass of C in 0.100g X = (12/44) x 0.309g = g 18g H 2 O contains 2g H mass of H in 0.100g X = (2/18) x 0.142g = g

Finding the formula of alkanes CH Ratio by mass : Ratio by moles : Simplest by ratio(÷ by smaller) 1:2.25 Whole number ratio 4:9 Empirical formula = C 4 H 9

Finding the formula of alkanes Example Relative molecular mass of X was found to be 114 by using a mass spectrometer Answer Empirical formula of X is C 4 H 9 but M r C 4 H 9 = 57 this is ½ of 114. So the molecular formula of X must be (C 4 H 9 ) 2 = C 8 H 18

Structure of alkanes Full structural formula of methane Shows all atoms and bonds Propane Shortened structural formula for propane Further shortened to

Structure of alkanes

Alkyl Groups Branches on carbon chains H H C CH 3 methyl H H H H C C CH 3 CH 2 ethyl H H

Naming organics – nomenclature Ref. P136 Organic Chemistry Solomons and Fryhle 2002 International Union of Pure and Applied Chemistry – IUPAC – has an internationally agreed and accepted method for giving evey organic molecule a unique name using a method - you are going to be examined on your ability to use this methodology. It is worth spending time learning and practicing this skill. Very useful indeed. The method is fairly easy and stepwise as follows:

Naming organics – nomenclature Ref. P136 Organic Chemistry Solomons and Fryhle Locate the longest continuous chain of carbon atoms; this chain determines the parent name for the alkane – this may not always be obvious since they go around corners!

Naming organics - nomenclature 2. Number the longest chain beginning with the end of the chain nearer the substituent

Naming organics - nomenclature 3. Use the numbers obtained from rule 2 to designate the location of the substituent group

Naming organics - nomenclature 4. When there are more than one group on different parts of the chain – number from the longest chain for example we call this “4-ethyl-2-methylhexane”

Naming organics - nomenclature 5. When two substituents are on the same carbon atom, use that number twice for example, this is 3-ethyl-3methylhexane

Naming organics - nomenclature 6. When two or more substituents are identical, indicate this by the use of the prefixes di-, tri-, tetra-, and so on. Make sure every substituent has a number. Commas separate numbers from each other. 2,3-dimethylbutane

Naming organics - nomenclature 7. Classification of hydrogen atoms 2-methyl butane has primary (1 o ), secondary (2 o ) and tertiary (3 o ) hydrogens

Naming organics - nomenclature 8. If there is a halide group – p140 – number from the first substituent attached to it regardless of whether it is halo or alkyl. If they are equal distance, then go alphabetically.

Naming alkenes - nomenclature 1. For alkenes, determine the parent by selecting the longest chain that contains the double bond and change the ending of the name from “–ane” to “-ene”

Naming alkenes - nomenclature 2. Number the chain so as to include both carbon atoms of the double bond and begin naming numbering at the end of the chain nearest the location of the double bond

Naming alkenes - nomenclature 3. Number the other substituents as we have already learned.

Naming alcohols – nomenclature (p141) In what is called IUPAC substitutive nomenclature a name may have as many as 4 features! Locant, prefix, parent compound and one suffix….(continuted)

Naming alcohols – nomenclature …For the alcohols, you add “-ol” to the suffix, in general, numbering of the chain always begins at the end nearer the group named as a suffix.

Naming alcohols – nomenclature (p141) 1. Select the longest chain again to which the hydroxyl (OH) group is attached. Change the name of the alkane corresponding to this chain by dropping the “e” and adding the suffix “ol”

Naming alcohols – nomenclature (p141) 2. Number the longest continuous chain so as to give the carbon atom bearing the hydroxyl group the lower number.

Naming Summary 1. Count the C’s in the longest chain 2. Name each attached group 3 Count the longest carbon chain to give the first attached group the smallest number 4. Name and locate each group

Naming Branched Alkanes CH 3 methyl branch CH 3 CH 2 CH 2 CHCH 2 CH Count 3-Methylhexane on third C CH 3 six carbon chain group

Branched alkanes -Isomers  Same molecular formula  Same number and types of atoms  Different arrangement of atoms

Cycloalkanes Cyclopropane CH 2 CH 2 CH 2 Cyclobutane CH 2 CH 2 CH 2 The is a group of alkanes that have a cyclic structure. These cycloalkanes contain a carbon chain that is in a ring. Each cycloalkane has a formula that is 2C less than the corresponding alkane. For example, propane is C 3 H 8 whereas cyclopropane ic C 3 H 6. Butane is C 4 H 10 and cyclobutane is C 4 H 10. The names of the cyclic structures use the prefix cyclo in from of the alkane name for the carbon chain

More Cycloalkanes Cyclopentane CH 2 CH 2 CH 2 CH 2 Cyclohexane CH 2

Shapes of alkanes represents a bond in the plane of the paper represents a bond in a direction behind the plane of the paper represents a bond in a direction in front of the plane of the paper Pairs of electrons in the bonds repel each other so in all covalent compounds the bonds are as far away from each other as possible. The bond angle for H-C-H is 109

Shapes of alkanes a simpler way of drawing ethane which shows the shape less accurately Ethane Each carbon atom is at the centre at of a tetrahedral arrangement

Shapes of alkanes Hydrocarbon chains are not really straight but a zig-zag of carbon atoms. All bond angles are 109 ◦

Reaction of alkanes Oxidation are v. unreactive unaffected by acids and alkalis and oxidising agents When they do react it is usually in the gas phase and energy needs to be supplied to get the reaction started

Reaction of alkanes Combustion alkane + O 2  CO 2 + H 2 O + heat C 6 H /2 O 2  6CO 2 + 7H 2 O + heat If air supply is limited combustion in incomplete and products include CO and C (soot) along with partially oxidised hydrocarbons

Action of heat on alkanes When alkanes fractions are heated under different conditions,3 different reactions can occur, isomerisation, reforming and cracking