Chapter 2. Alkanes. Table of Contents

Transcription

1 hapter 2 Table of ontents Introduction 1. Alkanes 2. Alkyl Groups 3. Nomenclature of Alkanes 4. Isomerism in Alkanes 5. Physical Properties of Alkanes 6. hemical Properties of Alkanes 7. Preparation of Alkanes 8. Methane 9. ycloalkanes Alkanes

2 hapter 2 Alkanes Warm up What does hydrocarbon word imply? Where do you use hydrocarbons in life? List down common names of hydrocarbons you commonly use.

3 hapter 2 Introduction

4 hapter 2 Introduction Hydrocarbons, as the name implies are compounds whose molecules contain only carbon and hydrogen. They are extracted from petroleum, natural gas and coal. Hydrocarbons Aliphatic Hydrocarbons 1. Saturated Hydrocarbons Alkanes(Parafins) 2. Unsaturated Hydrocarbons Alkenes(Olefins) Alkynes(Acetylenes) 3. ycloalkanes Aromatic Hydrocarbons (Benzene compounds)

5 hapter 2 Introduction

6 hapter 2 Introduction

7 hapter 2 Introduction Petroleum products

8 hapter 2 1. Alkanes ontain - and -H sigma bonds. Known as paraffin, meaning inert. General formula is n n+2, n is number of carbon atoms. First four members have common names, methane, ethane, propane and butane. Names end with ane suffix. In the names latin number + -ane is used, for example penta + ane = pentane - -, methylene, is difference between consecutive alkanes, forming homologous series.

9 hapter 2 1. Alkanes

10 hapter 2 1. Alkanes First 10 Members of Alkanes

11 hapter 2 1. Alkanes Straight chain alkanes take n- in front of name, meaning normal and shows the alkane is unbranched. H 3 n-butane H 3 H 3 n-hexane H 3 In branched alkanes, if one methyl,-h 3, group is on the second carbon the iso- prefix is added to name. H 3 H H 3 H 3 H H 3 H 3 iso-butane H 3 iso-pentane

12 hapter 2 1. Alkanes Additionally if there are two methyl,-h 3, groups are on the second carbon the neo- prefix is added to name. H 3 H 3 H 3 H 3 H 3 H 3 H 3 neopentane H 3 neohexane Example 1 What is the molecular formula of the alkane having the 58 g/mole molecular mass?

13 hapter 2 1. Alkanes Example g of an alkane, when analyzed, is found to contain 12 g carbon. What is its molecular formula? Example 3 If a 3 g sample of hydrocarbon is burned, 5.4 g of water vapor is produced. What is the molecular formula of hydrocarbon? Example 4 When the O 2 gas produced by the combustion 0.2 mol of an alkane is passed into limewater (a(oh) 2 ), 40 g of calcium carbonate precipitates. What is the molecular formula of the alkane?

14 hapter 2 2. Alkyl Groups If one hydrogen atom is removed from an alkane, an alkyl group is formed. General formula of alkyl groups is n n+1. Instead of the ane suffix in alkanes, -yl is used for naming of alkyl groups. They are represented as R, meaning radical. A carbon atom in an alkane can be named as primary, secondary or tertiary according to number alkyl groups bonded to it.

15 hapter 2 2. Alkyl Groups n n+2 n n+1 Alkane Alkyl Example 5 H H H H H H H methane methyl H 4 H 3 -

16 hapter 2 2. Alkyl Groups H R' R' R 1 o 2 o R R 3 o H primary Example 6 H secondary R" tertiary H 3 H 3 H 3 H 1 o 2 o 3 o 2 H 5 2 H 5 H primary H secondary H 3 tertiary

17 hapter 2 2. Alkyl Groups Example 7 Name the carbon atoms shown in the following molecules as primary, secondary or tertiary. H 3 H 3 H 3 H H 3 * * H 3 * 2o 3 o 1 o * H 1 o 3 H 3

18 hapter 2 3. Nomenclature of Alkanes For simple alkanes common names are used. H 3 H 3 H 3 n-butane H 3 H 3 H isopentane H 3 H 3 H 3 H tertiary butane For the branched complex alkanes IUPA system is used. In the IUPA system following rules can be used. 1. Determine the longest number of continuous carbon atom chain. This gives the parent name of alkane.

19 hapter 2 3. Nomenclature of Alkanes H 3 H 3 H 3 H H H 3 six carbon in the longest chain refers to hexane 2. Number the carbon atoms in the longest chain, starting nearer at the end nearer to the branched. H 3 H 3 H 3 1 H 2 H H 3

20 hapter 2 3. Nomenclature of Alkanes 3. Show the position of groups bonded to parent chain, if there are more than one identical groups, use di, -tri or - tetra prefixes. Some important groups are F (fluoro), -l (chloro), -Br (bromo), I (iodo), -NO 2 (nitro), -OH (hydroxyl), and N (amino) H 3 H 3 H 3 H H H 3 2, 4-dimethylhexane

21 hapter 2 3. Nomenclature of Alkanes 3. When two or more different groups are bonded to the carbon chain, they are ordered alphabetically. H 3 l H 3 H H H H 3 2 H 5 4-chloro-3-ethyl-2-methylhexane

22 hapter 2 3. Nomenclature of Alkanes

23 hapter 2 3. Nomenclature of Alkanes Example 8 Name the following structures. a. b. H 3 d. H 3 H 3 H H 3 Isobutane or 2- methylpropane H 3 H 3 H 3 H 3 H 3 H 3-methylpentane e. H 3 Br H 3 2 H 5 3-ethyl-3-methylhexane 3 H 7 4-bromo-4-ethylheptane

24 hapter 2 3. Nomenclature of Alkanes Example 9 Draw the structures of following alkanes. a. 3-ethyl-2-methylpentane b. 1,3-dibromopropane c. 2,2,3,3-tetramethylbutane d. 2,2,4-trimethylpentane e. 2-chloro-2,3-dimethyl-3-ethylheptane f. 5-ethyl-2-methylheptane

25 hapter 2 4. Isomerism in Alkanes ompounds having same molecular formula but different structural formula are said to be isomers. First three members of alkanes do not have isomers. Physical and chemical properties of isomers are different. H 3 H 3 H H H H n-butane 2-methylpropane (isobutane) Example 10 Write the structural formulae and the IUPA name of possible isomers of hexane, 6 H 14.

26 hapter 2 4. Isomerism in Alkanes

27 hapter 2 Alkanes are non polar molecules, only van der waals forces are responsible for the attractions between molecules. 1-4 are gases, 5-17 are liquids, and the rest are solids. Bp, Mp, and densities of alkanes increase by the increasing number of carbon atom. They are generally tasteless, odorless and colorless. Branching decreases boiling point. 5. Physical Properties of Alkanes They are insoluble in water, but soluble in organic solvents such as benzene and carbon tetrachloride.

28 hapter 2 Another name for alkanes is paraffin, meaning inert. They are chemically quite unreactive. 6. hemical Properties of Alkanes They do not react with dilute acids (Hl, SO 4 ), active metals(na, K) and strong oxidizing agents(kmno4, Na 2 r 2 O 4 ). They only undergo combustion and substitution reaction with halogens. They can be nitrated under high pressure. 1. ombustion Reaction n n+2 + (3n+1)/2 O 2 n O 2 + (n+1) O H 4 + 2O 2 O O

29 hapter 2 6. hemical Properties of Alkanes

30 hapter 2 2. Substitution Reaction Light 6. hemical Properties of Alkanes R H + X X R X + H X Halogenetion alkane halogen heat alkyl halide hv H 4 + l 2 H 3 l + Hl 425 R H + HO NO 2 o R NO 2 + O Nitration Alkane nitroalkane

31 hapter 2 2. Substitution Reaction 6. hemical Properties of Alkanes

32 hapter 2 6. hemical Properties of Alkanes 3. racking Reactions Higher alkanes are converted into smaller alkanes and alkenes at high temperature. When a catalyst is used in the process, it is known as catalytic cracking. 4. Isomerization Reaction Straight-chain alkanes are converted into branched alkanes. H 3 n-hexane H 3 heat All 3 H 3 H H 3 isohexane H 3

33 hapter 2 6. hemical Properties of Alkanes Example 11 When 8.8 g of an alkane is burned completely, 26.4 g of carbon dioxide and 14.4 g of water are produced. What is the molecular formula of the alkane? Example 12 To burn a mixture of 33.6 L of methane and propane gas at STP, 144 g of oxygen gas are required. According to this information, what is the total mass of the original mixture? (H:1, :12, O:16 ) Example 13 What is the pressure of O 2 gas in 2-L container which is produced from the combustion of 7.2 g of 5 H 12 at 27 o?

34 hapter 2 7. Preparation of Alkanes Main sources of alkanes are crude oil, natural gas and coal. Many useful alkanes are obtained from fractional distillation of petroleum. 1. Wurtz Synthesis This method was first used by Adolph Wurtz in Methane cannot be produced in this process. R-X + 2Na + X-R R-R + 2NaX 2H 3 -Br + 2Na H 3 -H 3 + 2NaBr

35 hapter 2 7. Preparation of Alkanes Example 14 Which alkyl halides can be used to prepare pentane by the Wurtz synthesis. Example 15 What is(are) the products of the reaction of 2-chloropropane and 1-bromoethane with excess sodium metal?

36 hapter 2 7. Preparation of Alkanes 2. Hydration of Grignard ompounds R-X + Mg Anhydrous Ether R-MgX Grignard Reagent H-OH R-H + Mg(OH)X Alkane This method was first used by Victor Grignard in Example 15 Prepare ethane by using Grignard reagent. Solution H 3 Br + Mg Anhydrous Ether H 3 MgBr Ethyl Magnesium Bromide Grignard Reagent H 3 MgBr + H-OH H 3 H 3 + Mg(OH)Br Ethane

37 hapter 2 7. Preparation of Alkanes 3. Reduction of Alkyl Halides Zn 2R - X 2R-H + ZnX 2 alkyl halide H + alkane Example 16 Prepare propane by using 2-bromopropane. Solution 2H 3 HH 3 Br 2-bromopropane Zn H + 2H 3 H 3 + ZnBr 2 propane

38 hapter 2 7. Preparation of Alkanes 4. Heating the Metal Salts of arboxylic Acids Heat R-OONa + NaOH R-H + Na 2 O 3 sodium salt of ao Alkane a carboxylic acid Example 17 Prepare methane by using sodium acetate. Solution H 3 -OONa + NaOH Sodium Acetate Heat ao H 4 + Na 2 O 3 Methane

39 hapter 2 7. Preparation of Alkanes 5. Hydrogenation of Unsaturated Hydrocarbons + Pt, Pd or Ni alkene H H alkane alkyne + 2 Pt, Pd or Ni H H H H alkane

40 hapter 2 7. Preparation of Alkanes Example 17 5 mol mixture of 3 H 6 and 3 H 8 is reacted with 2 mol of gas. What is the total volume of gases at STP when the reaction is completed? Solution In the mixture only 3 H 6 reacts with. Ni 3 H H 8 2 mol 2 mol 3 H 6, the mol of 3H8 = 5 2 = 3 mol is unreacted. nt = 2 mol 3 H 8 produced + 3 mol 3 H 8 unreacted = 5 mol V= 5x22.4 = 112 L

41 hapter 2 8. Methane It is found in mines and in natural gas. It is also known as marsh gas and is formed by decomposition of plants in the absence of oxygen. It is colorless and odorless has. It is insoluble in water, but soluble in benzene and gasoline % of methane in air may cause an explosion. It is called in mines firedamp explosions. It burns with a light blue flame.

42 hapter 2 8. Methane 1500 o H o H H 4 + O 2 Preparation catalyst 260 o 100 atm 2H 3 OH H + Al O 4Al(OH) 3 + 3H 4

43 hapter 2 8. Methane Ni O + 3 O + H o Ni + 2 H 4 pressure, heat Example 18 What is the percent of a 150 g of Al 4 3 sample that produces 67.2 L of H 4 gas when put in water? (Al:27, H:1, :12)

44 hapter 2 9. ycloalkanes Alkanes bonded to one another in a ring formation is called cycloalkanes. General formula is n n. Nomenclature of ycloalkanes Use cyclo- prefix to the names of alkanes. H 3 propane H 3 cyclopropane cyclopropane

45 hapter 2 9. ycloalkanes H 3 butane H 3 cyclobutane cyclobutane H 3 l 2 H 5 Methylcyclobutane hlorocyclopentane Ethylcyclohexane If there are more than one substituent in the ring, the ring is numbered starting with the substituent first in the alphabet, and numbered in the direction which gives the lowest number for the second substituent.

46 hapter 2 9. ycloalkanes l 1 2 H 3 H 3 2 H chloro-2-methylcyclobutane H 3 1-ethyl-3,4-dimethylcyclohexane If the number of carbon atom in the ring is less than that is in the substituent, then they are named as cycloalkylalkanes. H 3 H 3-cyclopropylhexane H 3 H 3 1-cyclobutylpentane

47 hapter 2 9. ycloalkanes

48 hapter 2 9. ycloalkanes Physical Properties Melting point, boiling point and densities of cycloalkanes are different from the alkanes possessing the same number of alkanes. Melting and boiling points increase as the number of carbon atom increases in the ring. hemical Properties ycloalkanes with more than 5 carbon atom undergo substitution reaction. ycloalkanes with 3 or 4 carbon atoms undergo addition reaction.

49 hapter 2 9. ycloalkanes

50 End of the chapter 2