Compounds of Carbon

1. Compounds of Carbon Chapter 8

2. Why is carbon important? • Carbon makes up over 90% of all chemical compounds • They form the basis of living systems • Carbohydrates all have carbon • Proteins contain carbon • Fats contain carbon

3. How does carbon form so many compounds? • Carbon has 4 valence electrons, all available for bonding with other atoms • Carbon can form strong covalent bonds with other carbon atoms • Bonds between carbons can be single or multiple

4. Hydrocarbons • Hydrocarbons are made up of different compounds of hydrogen and carbon. • There are many different hydrocarbons • They make up the majority of the petroleum and natural gas industry • Hydrocarbons can be classified into several families or homologous groups. • The simplest hydrocarbon is methane and it belongs to the alkane series.

5. Homologous groups • A series of compounds with similar properties in which each member differs form the previous one by –CH2- is called a homologous group • Members of the same homologous groups tend to have very similar chemical properties. So organising carbon compounds into homologous series simplifies the study of hydrocarbons.

6. Alkanes • Alkanes consist of carbon and hydrogen only. • They contain only single bonds • Look at the table, each alkane differs by –CH2- • The alkanes have a general formula of CnH2n+2 • If a compound has 16 carbons, then 2 x 16 + 2 = 34 • So it would have the fomula C16H34

7. Representing alkane molecules • When drawing hydrocarbons we use structural formulas • These are very similar to valence structures except they don’t show the unbonded pairs. • In structural formulas we focus on the location of the atoms relative to one another in the molecule as well as the number and location of chemical bonds.

8. The above diagram shows the first three alkanes. • You will notice: • Each carbon atom forms a single covalent bond to four other atoms • Each hydrogen atom forms a single covalent bond to one carbon atom • The four atoms bonded to each carbon atom are arranged in a tetrahedral shape.

9. Your Turn • How would we draw the structural formula for C4H10

10. There are two possible ways. • The first has the four carbon atoms in a continuous chain. The overall molecule is said to be linear. • These are called straight-chain molecules • The second one is not linear, this is called a branched chain molecule

11. Isomer • Molecules which have the same chemical formula but can form different arrangements of their atoms are called isomers. • Same number of atoms just arranged differently. • Structural isomers have similar chemical properties but can differ in some physical properties such as melting and boiling temperatures

12. Alkanes • The alkane series contains only single bonds. • The alkanes are known to be saturated hydrocarbons as the carbons are saturated with hydrogens • Meaning each carbon is completely bonded to either hydrogen or carbon, there are no unbonded carbons.

13. Alkenes • Alkenes contains one double bond between two carbons. • Like alkanes, alkenes also differ by one –CH2- group. • The alkenes also form a homologous series. • The alkenes generally have the formula CnH2n.

14. Representing Alkene Molecules • Like ethene, propene C3H6 also has one carbon-carbon double bond.

15. Butene • Butene (C4H8), like butane has more than one isomer. • The alkenes are classified as unsaturated hydrocarbons. The double bond means that alkenes contain less hydrogen than the maximum amount possible.

16. Semistructural formulas • When we want to summarise the structural formula without indicating the 3D arrangement we use semistructural formulas. • The semi structural Formula for propene is C3H6

17. Your Turn • Page 140 • Questions 3 - 6

18. Naming Carbon Compounds • How do we name carbon compounds? • How do we distinguish between structural isomers? • There are a set of rules put in place by which chemists can derive a systematic name for a given compound.

20. Straight-chain hydrocarbons • The first part of the name refers to the number of hydrocarbons. • The second part refers to type of bonds • ane if all carbon-carbon bonds are single • ene if one C-C bond is a double • yne if one C-C bond is a triple • Pentane, pentene and pentyne all have 5 carbons bonded in a linear or straight chain.

21. Unsaturated compounds • Unsaturated hydrocarbons contain at least one multiple bond. • Butene has three isomers, two of which are straight chained, as the carbon chain becomes longer the number of isomers increases. • To name straight-chain alkenes, first number the carbon atoms in the chain, starting with the end that will give the first carbon atom involved in the double bond the smallest number possible.

22. The number starts at the end closest to the double bond. • The isomer is named according to the first carbon atom involved in the double bond. • The first isomer is but-1-ene. • The other isomer is but-2-ene

23. Branched Hydrocarbons • An alkyl group usually forms a branch in a branched chain hydrocarbon. • An alkyl group is an alkane molecule less on hydrogen atom • It is named after the alkane from which it is derived. • -CH3 is a methyl group. • -C2H5 (-CH2CH3) is an ethyl group

24. Branched Hydrocarbons • Systematic naming requires us to: • Identify the longest continuous chain of carbon atoms in a molecule. • Identify the side group that forms the branch in the chain • Number the carbon atoms from one of the ends of the longest carbon chain so that the side group is attached to the carbon atom with the smallest possible number.

25. C4H10 • The longest chain of carbons has 3 carbons and all the bonds are single. • Therefore the molecule is derived from propane. • Identify the side group • The side group is a methyl group. • Number the carbons. The methyl group is on the second carbon. • The compound is therefore 2-methylpropane.

26. Worked Example 8.4, page 143

27. Some are done for us • Take a look at page 144 of your text. • Which are the alkanes? • Which are the alkenes?

28. Functional Groups • -CH3 – methyl • -OH – alkanol • -Cl (or F, or B or I) chloro (or fluoro etc) • -NH2 - amino

29. Your Turn • Page 147 • Questions 7 and 8

30. Chemical Properties of Alkanes • The most significant reaction of alkanes is combustion. Alkanes burn in oxygen, releasing large quantities of energy. • If the oxygen supply is sufficient the products released are carbon dioxide and water. • This energy released is what we use as a source of heat, to produce electricity for domestic and industrial use.

31. Equations for Combustion of Reactions This figure shows the rearrangement of atoms that occur when the hydrocarbon methane burns in oxygen. Have the atoms of each element been conserved? The equation is CH4 + 2O2→ CO2 + 2H2O

32. Chemical Properties of Alkenes • Due to the double bond in alkenes they react much more readily and with more chemicals than the alkanes. • Alkenes, in particular, ethene and propene, are not used for fuels but rather as starting materials to manufacture a huge range of compounds such as alcohols, antifreeze and plastics. • Apart from combustion, the reactions of alkenes usually involve the addition of a small molecule to produce a single product.

33. Addition reactions of ethene Reaction with Bromine solution • Ethene reacts with bromine solution as shown. • In addition reactions, bonding “new” atoms to the two carbons on either side of the double bond, converts the C=C double bond to a C-C single bond.

34. Reaction with Steam • Large amounts of ethanol are now made by the addition of steam and ethene using a phosphoric acid catalyst. • This ethanol is used as a reagent for industrial purposes and as a solvent in cosmetics and pharmaceuticals. • This is not the ethanol that people drink.

35. Formation of polyethene • An addition reaction of ethene is involved in making polyethene. • As seen previously the C=C bond is converted to a C-C bond and a saturated product is formed. • In this case there is no other reactant to add to the ethene molecules. Polyethene is formed when ethene molecules themselves join together to form a long chain.

36. Polyethene is usually written as (-CH2-CH2-)n • Where n is a large number • A molecule made by linking a large number of small molecules such as ethene is called a polymer (meaning many units). Each small molecule is called a monomer (one unit). • This type of reaction is addition polymerisation

37. Your Turn • Page 151 • Questions 9-10

38. Polymers • Polymers are long chained molecules • Each one can contain tens of thousands of atoms. • Cotton, wool and silk are some naturally occurring polymers. • Synthetically made polymers are generally superior to natural polymers as they have been designed for specific properties.

39. Synthetic polymers Include: • Cling wrap • Drugs • Clothing • Domestic appliances • Cars • Sporting equipment Check out table 8.8 on page 153

40. Not Plastic • We frequently use the term plastic when referring to polymers. • The term “plastic” refers to the property of a material not the material itself. • A substance is “plastic” if it can be moulded into different shapes easily. • Many polymers are indeed plastic, some however are not. • The materials used to make powerpoints are brittle and cannot be reshaped.

41. What are polymers? • Polymers are large covalently bonded molecules. • They contain tens of thousands of atoms • They are formed by joining together smaller molecular units called monomers. • The size of the polymer varies, a polymer can consist of varying sizes of molecules formed from different numbers of monomers.

42. Polymers • There are two types of polymerisation processes. • Addition polymerisation • Condensation polymerisation • The polymers formed by addition polymerisation often have the monomer included in the name of the polymer. • Polyethene is formed by the monomer ethene. • Condensation polymers are named after the chemical bond they form. • Polyesters contain monomers joined by an ester functional group

43. Addition Polymers • Most polymers are built around atoms of carbon like their monomers. • Covalent bonds form between the monomers to produce a polymer molecule.

44. Addition Polymers • Suitable monomers for addition polymerisation are unsaturated molecules. • What are unsaturated molecules? • The double bond between the two carbon atoms react and new covalent bonds are formed between carbon atoms on nearby molecules creating long chains.

45. Polyethene • Read page 155 – 156. • Why is High-density polyethene stronger and more rigid than low-density polyethene?

46. Structure, properties and applications • Two very important properties of polymers are tensile strength and softening temperature. • Tensile strength is a measure of the materials resistance to breaking under tension. It determines the structural uses of the polymer. • The softening point affects the way the polymer can be moulded. • Both tensile strength and softening point are determined by the strength of the forces between polymer chains.

47. Thermoplastics • Thermoplastics are polymers that can be moulded and shaped. • The tensile strength and softening point are affected by: • Degree of branching • Nature of atoms of groups of atoms attached to the carbon chain • How the atoms or groups of atoms are arranged along the chain.

48. Cross-linking • Another factor that affects the properties of a polymer is cross-linking. A cross-link is a covalent bond between polymer chains. • The more cross-links the stronger and rigid the polymer. • The strong covalent bonds in 3D bind all the atoms together to form one large lattice.

49. Thermosets • Thermosets are polymers with extensive cross-linking. • They do not soften on heating as thermoplastics do. • When heat is applied the covalent bonds break and the thermosetting polymer will decompose rather than soften.

50. Degree of branching • Low denisty polyethene contains a higher degree of branching which lowers the density, hardness and melting point of a polymer. • Low denisty polyethene is more flexible and is used in cling wraps and squeeze bottles. • High density polyethene is harder and less flexible, used in pipes and toys