Understanding Homologous Series in Chemistry

1. Unit 2 Nature's Chemistry Homologous Series

2. How do you know these people are related? Who stands out? Why?

3. How do you what you are buying? What stands out? Why?

4. Homologous Series (family) • Each homologous series has a general formula. • All members possess the same functional group. It is the functional group that gives the homologous series its characteristic reactions. • Members of the same homologous series have similar chemical properties.

5. Homologous Series (family) • There is a gradual change in physical properties e.g. melting and boiling points of the alkanes increase as molecular size increases (see data book p 9). Why? • Attractive forces between the molecules (intermolecular forces) become larger as molecular size increases -> more energy is needed to separate the molecules -> higher m.pt. and b.pt. .

6. Question – How many different ways can be count up to 8? Number of carbons in the hydrocarbon compound; One = Meth Five = Pent Two = Eth Six = Hex Three = Prop Seven = Hept Four = But Eight = Oct

7. Prefixes M onkeys (meth... 1 carbon) E at (eth… 2 carbons) P eanut (prop… 3 carbons) B utter (but… 4 carbons) P andas (pent… 5 carbons) H ate (hex… 6 carbons) H airy (hept… 7 carbons) O ranges (oct… 8 carbons)

8. C H n 2n+2 Alkanes Important characteristics of the alkane homologous series; • The names of all alkanes end in ‘ane’ • All bonds are single (saturated) • All alkanes follow the general formula; Important

9. Isomers and Naming Isomers have the same chemical formula(same amount of ‘stuff’) but a different structural formula (‘different layout’) For the first three members of the alkanes only one structure is possible but butane, C4H10, has two isomers; white board example

10. Rules for naming branched alkanes; • Find the longest carbon chain in the molecule – the name is based on the alkane with this number of carbon atoms. 2. Number the chain to give any branches the lowest possible number(s). 3. Name the branches: methyl (-CH3), ethyl (- C2H5), propyl (-C3H7) etc.

11. Worked example; Draw and name the isomers of C5H12 white board example

12. Reactions of Alkanes The alkanes are fairly unreactive however they do burn well – therefore their main use is in fuels; • Combustion Example white board example

13. Cycloalkanes Characteristics • The names of all cycloalkanes start with cyclo- and end in –ane. • All bonds in the cycloalkanes are single. (saturated) • Cycloalkanes are ring structures • First member of the series is cyclopropane. • Used in motor fuel, kerosene, diesel, and many other heavy oils. • All cycloalkanes follow the general formula; C H Important n 2n

14. Alkenes Important characteristics of the alkene homologous series; • The names of all alkenes end in ‘ene’ • At least one C=C double bond (unsaturated) • Therefore 1st member is ethene. • All alkenes follow the general formula; C H Important n 2n

15. Isomers and Naming When naming alkenes we follow the same rules as for the alkanes with one major difference; • the main chain (the longest chain) must contain the double bond, whose position is indicated by a number. • The chain is numbered to make this number as small as possible(the double bond takes priority over any branches.)

16. Draw and name the isomers of C5H10 white board examples x6

17. Saturated and Unsaturated Hydrocarbons are described as either saturated or unsaturated. ____________ and ____________ are saturated. They have no double (or triple) carbon to carbon bonds. Every carbon atom is linked to neighbouring atoms by single bonds only (C-C) ________ are unsaturated hydrocarbons. They are unsaturatedbecause each molecule contains at least onecarbon to carbon double bond (C=C) Alkenes are more reactive than the alkanes and cycloalkanes because of the carbon to carbon double bond. alkanes cycloalkanes alkenes

18. Test for Unsaturation The test for unsaturation is a test to see if a molecule has a double or triple bond. Bromine is used to test for unsaturation as it rapidly decolourises when added to an unsaturated compound i.e. if bromine changes colour from orange to colourless you have an alkene. This is useful for identifying an alkene and a cycloalkane from one another if they have the same chemical formula.

19. Addition Reactions During the test for unsaturation the bromine ‘adds on’across the double bond, giving a saturated compound. This is an example of an addition reaction. e.g. Propene + Bromine dibromopropane white board example

20. Another Example; Pentene + Bromine dibromo_________ Bromine = bromo… Chlorine = chloro… Fluorine = fluoro… Iodine = iodo… Nitrogen = nitro… extra - not needed for exam

21. Another Example; Heptene + Hydrogen _________ An addition reaction that involves hydrogen is called hydrogenation

22. Butene + Oxygen This is combustion not addition... All hydrocarbons burn to produce carbon dioxide and water

23. Ethene+ Water An addition reaction that involves water is called hydration All hydrocarbons burn to produce carbon dioxide and water

24. Consumer Products

25. Alcohols • The names of all alcohols end in ‘-ol’ • The functional group of the alcohols is –OH(hydroxyl group) • Alcohols are substituted alkanes (as an -H has been replaced with an –OH) • Alcohols are made via hydration of alkenes and fermentation. • Alcohols make useful industrial solvents. • All alcohols follow the general formula; C H O H n 2n +1

26. Isomers and Naming Isomers can result from both branching and varying the position of the -OH group. In naming, the main chain (longest chain) must contain the -OH group, whose position is indicated by a number. Example; Pentan-1-ol, Pentan-2-ol, Pentan-3-ol White board Example

27. Example 2 – Draw the 4 isomers of C4H9OH White board Examples

28. Alcohols as Fuels Alcohols make really good fuels and burn with a clean flame. A reaction (like combustion) which gives out heat to the surroundings is classed as exothermic. The opposite of an exothermic reaction is an endothermic reaction (i.e. heat is taken in.) White board Example

29. Carboxylic Acids • The names of all carboxylic acids end in ‘-oic acid’ • The functional group of the carboxylic acids is -COOH (carboxyl group) • This carboxyl group is always attached to the end carbon. • Carboxylic acids react like other acids • General Formula; Cn H2n+1 COOH

30. Vinegar • Vinegar is dilute ethanoic acid. • Vinegar has been used for years in cooking both as a flavouring and as a preservative. • Vinegar is also a useful cleaning agent and is found in many household products.

31. Fuels Calculation We can calculate the energy released by a fuel using the following equation; Eh = c x m x ΔT where: c = Specific heat of water, 4.18 kJ kg−1 °C−1 (data book) m = Mass of water in kg ΔT = Temperature change of water in °C Eh = Energy (kJ)

32. Worked Example 1 Calculate the amount of energy produced when 0.5g of methanol is burned to heat 250cm3 of water. The water temperature went up by 2°C. White board Example

33. Worked Example 2 Calculate the amount of energy produced when 200g of methane is burned to heat 50cm3 of water. The initial temperature of water was 25°C and at the end of the experiment the temperature was found to be 65°C. White board Example

34. Worked Example 3 Calculate the amount of energy produced from the following results; Methanol mass before 53.65g Methanol mass after 53.46g Mass of water heated 100g Temp rise in water 10°C White board Example