Chapter 4 - Carbon Compounds

Transcription

1 Chapter 4 - Carbon Compounds Carbon compounds organic compounds are tied up with living organisms. So much so, that as we have seen, the presence of methane might be considered an indicator of life. Methane is one end of the organic spectrum as it represents a fully reduced carbon (CH 4 ). At the other extreme is carbon dioxide, a fully oxidized carbon (C 2 ). In between those extremes, there are many other compounds with a single carbon, such as chloromethane ( Cl), methanol ( H), formaldehyde (methanal, H 2 C=), and formic acid (methanoic acid, HCH). Chapter 4 1

2 To start our understanding of organic chemistry, or to continue what we started in the last chapter, we need to start with organic nomenclature. The simplest organic compounds consist of only carbon and hydrogen and are called hydrocarbons : Chapter 4 2

3 The simple alkanes are built from H-(CH 2 ) n -H or with a general formula of C n H 2n+2. The name of the alkane chains is the root of naming. Chapter 4 3

4 Generally speaking, - each alkane molecule has a framework of tetrahedral carbon atoms - each carbon atom has four single bonds (to other carbon atom(s) and to at least two hydrogens) - free rotation occurs around the carbon-carbon single bonds In drawing alkane molecules, - create the framework of carbon atoms in the chain - fill in the remaining positions with hydrogen atoms so that each carbon has four bonds - draw the atoms to ensure that the chain is obvious orientation is not as important Chapter 4 4

5 Drawing alkanes: Chapter 4 5

6 If straight chain molecules are all that we could have for alkanes, there would still be many organic compounds, but not as many as we have. However, starting at four carbons, we have the possibility of different connectivity patterns: C 4 H 10 : Isomers have the same number of atoms but arranged in a different fashion. n-butane and isobutane are called constitutional isomers they are constituted differently. Chapter 4 6

7 To name constitutional isomers, we could try to come up with a unique name for each one. But consider that for C 20 H 42 there are a 1000 or so isomers, it would be impossible to name them all! Instead, we take a systematic approach: - identify the longest chain and name it as the corresponding alkane - identify the substituent groups attached to the chain - group substituents together where possible - number the chain from the end that gives the lowest possible set of numbers for the substituents Chapter 4 7

8 Chapter 4 8

9 Some examples: Chapter 4 9

10 Some examples: Chapter 4 10

11 Some examples: Chapter 4 11

12 rganic compounds are found throughout the biosphere all life, as we know it, is built around carbon. Not surprisingly, it is fairly easy to find organic compounds. Everything from DNA to cellulose to proteins are made of carbon based compounds: Chapter 4 12

13 Indeed, organic compounds pervade even the air in the environment around us. Forest haze is a result of alkane emissions by trees. Chapter 4 13

14 However, most of our organic compounds find their origin in petroleum. Everything from birth control pills to plastic pens, along with gasoline, jet fuel, and asphalt originates with petroleum. The first step in refining petroleum or crude oil is distillation which separates the oil into its respective fractions based on boiling points. Chapter 4 14

15 Alkanes whether straight chain or branched - are not the only organic compounds. Sticking to just hydrocarbons, for now, the other possibilities include: Chapter 4 15

16 Alkenes all have C=C double bonds. (They are unsaturated.) This effects the way that we name compounds because we have to indicate the presence of the double bond. We do this by using the root for the corresponding alkane and substituting the -ene for -ane. Hence: - ethane becomes H 2 C=CH 2 ethene (aka ethylene ) -CH 2 - propane becomes H 2 C=CH- propene But note that with butene we have two possibilities. The double bond can occur in two different locations: H 2 C 1-butene 2-butene Chapter 4 16

17 The general approach for naming alkenes and other organic compounds can be summed up with the following diagram: That is, 2-butene has a: prefix: 2 telling us that the double bond starts on the second carbon and extends to the third parent: -but- telling us that there are four carbons in the chain in total suffix: -ene telling us that there is one double bond in the chain Chapter 4 17

18 But this isn t the only consideration with double bonds. While fragments of molecules are capable of free rotation around single bonds, the same isn t true of double bonds. Double bonds have restricted rotation: Chapter 4 18

19 There is another system of nomenclature that orders the groups across a double bond: The advantage of this system is that it works for a molecule which has four different groups, where it isn t totally obvious whether it is cis or trans. It is also easier to write. Chapter 4 19

20 Further, Hence, this compound is 2-ethyl-1-pentene or 2- ethylpent-1-ene. The latter is the preferred method of writing the name as it explicitly labels the double bond as being in the first position. Note that this is neither cis nor trans as the groups at one end of the double bond are exactly the same both hydrogen atoms. Chapter 4 20

21 As an example of the importance of cis and trans, consider the reaction of retinal. This is the molecule in your eye that absorbs light, and allows you to see. The fact that you can read this slide is because this molecule undergoes a conversion from cis to trans! Chapter 4 21

22 What if a molecule contains more than one double bond? Need to number each of the bond and indicate the total number in the name. CH 2 1,3-butadiene H 2 C CH 2 H 2 C 2-methyl-1,3-butadiene or 2-methylbuta-1,3-diene penta-1-cis-3-diene H 2 C Chapter 4 22

23 Chapter 4 23

24 It is also possible for a compound to have a CΞC triple bond. These hydrocarbons are called alkynes and the -yne ending is used in naming in the same way that the -ene ending is used for double bonds. The simplest alkyne is ethyne but it is invariably referred to as acetylene : HC CH Propyne: HC 1-Butyne: HC 2-Butyne: Note that since there is only one substituent at each end of a triple bond, there is no cis or trans (or E or Z ). Chapter 4 24

25 Another possibility is that the carbon chain will form a ring called a cycloalkane. It is an alkane because all of the carbon-carbon bonds are single bonds but it is important to realize that all cycloalkanes have a general chemical formula of C n H 2n which is the same general formula as observed for alkenes. This leads to a ring being a double bond equivalent. Chapter 4 25

26 Cyclohexanes have a special place in organic chemistry because six membered rings are pervasive. Consider the steroid framework: For example, cholesterol : Chapter 4 26

27 Examples: Chapter 4 27

28 Chapter 4 28

29 As a final point, cycloalkanes can also contain double bonds. The number system is straightforward in that the double bond takes precedence and the -ane ending is converted to -ene. Hence, cyclohexene cyclohexa-1,4-diene cyclohexa-1,3-diene benzene Chapter 4 29

30 Hydrocarbons are not the only possibility for organic compounds. ther elements can be found in organic compounds. Two are very prevalent oxygen and nitrogen forming a wide variety of compounds. This array of compounds can be organized around the functional groups present in the molecule. Chapter 4 30

31 Alkyl Halides: The halogens (F, Cl, Br, and I) are capable of replacing hydrogen within a hydrocarbon molecule. Essentially, the halogens form single bonds and look like hydrogen from a bonding point of view. C H 3 F fluoromethane F Cl F Cl dichlorodifluoromethane I C H 3 Br Br 2,2-dibromopropane Cl Cl F 4-iodoheptane Br Chapter 4 31 Br Cl 1,1-dibromo-2,4,4-trichloro- 3-fluorocyclohexane

32 Alcohols: Any hydrogen atom in a hydrocarbon can be replaced by an -H group, with the resulting compound being an alcohol. Alcohols are named by replacing the last e in the name with -ol. These can be represented by RH where R is any organic radical a molecular fragment or substituent. The simplest alcohol is generated from methane: Methanol H (aka wood alcohol ) The one that most people enjoy is ethanol ( CH 2 H) also known as ethyl alcohol or just alcohol. Chapter 4 32

33 With propane, there are now two possibilities: C H 3 With butane: 1-propanol H H 2-propanol (aka isopropyl alcohol ) C H 3 1-butanol H H 2-butanol C H 3 H isobutyl alcohol (2-methyl-1-propanol) H t-butyl alcohol (2-methyl-2-propanol) Chapter 4 33

34 With a double bond: H pent-3-en-2-ol n a cyclic compound: H H 3-methyl-cyclohexanol H cyclopenta-1,2-diol n a benzene ring: H phenol Chapter 4 34

35 Carbonyls Aldeydes and Ketones: A step up from alcohols are the carbonyl compounds which feature a C= double bond. Aldehydes feature a hydrogen attached to the carbonyl. Ketones have two R-groups. R H R R Aldehydes -use -al instead of ol for endings Ketones -use -one instead of -ol for endings Chapter 4 35

36 Another way to think of an aldehyde is that it is an organic compound containing a C= at the end of a hydrocarbon chain. H H formaldehyde (methanal) C H 3 H acetaldehyde (ethanal) H propanal H H 2-methylpentanal benzaldehyde Chapter 4 36

37 A ketone is an organic compound containing a C= at a position other than at the end of a hydrocarbon chain. The simplest possible ketone requires three carbons: acetone (2-propanone) ther ketones: 2-hexanone 3-methyl-2-hexanone cyclohexanone acetophenone Chapter 4 37

38 Ethers: An ether is a compound in which an oxygen bridges two alkyl groups. It is similar, in structure, to an alcohol, except instead of RH, it is R R. Ethers can be named in two ways as di-alkyl ethers or as alkoxide radicals attached to a carbon chain. diethyl ether ethoxyethane Chapter 4 38

39 Examples: C H 3 1-methoxypropane (methyl propyl ether) C H 3 CH 3 1-methoxy-3,3-dimethylbutane 2-methoxypropane (methyl isopropyl ether) 1,2-diethoxycyclohexane CH 3 epoxide 2-methoxy-3,3-dimethylbutane Chapter 4 39

40 Amines: An amine is an organic compound containing an -NH 2 group or, maybe more accurately, a nitrogen atom. Because of the nitrogen, amines are organic bases, reacting with acids to ammonium ions (-NH 3+ ). NH 2 aminomethane (methyl amine) NH 2 aminoethane (ethyl amine) NH diethyl amine (3-azapentane) NH 2 H 2 N 1,4-diaminobutane (putrescine) H 2 N NH 2 1,5-diaminopentane (cadaverine) N triethyl amine (3-ethyl-3-azapentane) Chapter 4 40

41 Carboxylic Acids: rganic acids contain the functional group -CH. Note that this is always the highest priority group when naming and must be at the end of the molecular chain. The R CH group is: R H where the carbon atom is high oxidized. Note that deprotonation of the acid leads to the carboxylate ion (RC - ): RCH RC - + H + Chapter 4 41

42 Examples: H H H H methanoic acid (formic acid) ethanoic acid (acetic acid or vinegar) butanoic acid (butyric acid) H 2 N H H H 3-aminobutanoic acid 3-oxobutanoic acid (acetoacetic acid) benzoic acid Chapter 4 42

43 Carboxylic Acid Derivatives: There are a number of compounds that can be viewed as carboxylic acid derivatives that is, compounds where the -H or the -H of the carboxylic acid group has been replaced by another functional group. R R R esters R' amides NHR' X acid halides X=F, Cl, Br, I R R' acid anhydrides where R and R may be the same or different alkyl groups Chapter 4 43

44 Amides: An important functional group in biological systems. There are 20 naturally occurring amino acids which have a structure based on: Amino acids can link through amide groups to give proteins: Chapter 4 44

45 Esters: An ester contains a RCR functionality, which means that it is no longer acidic. Esters form from the reaction of alcohols (R H) and acids (RCH): RCH + R H RCR + H 2 which occurs in many natural systems. Unlike acids, which have sharp odours, esters frequently produce pleasant odours. Ester dour Ester dour methyl butanoate pineapples pentyl propanate apricots pentyl ethanoate bananas ethyl methanoate rum octyl ethanoate orange rind ethyl butanoate apple Chapter 4 45

46 Esters are named based on the acid and alcohol from which they are made. The alcohol is converted to an alkyl radical (for example, methanol becomes methyl ) and the -oic acid is converted to an -oate. Examples: ethyl acetate C H 3 methyl butanoate C H 3 butyl methanoate Note that fats are esters triesters, in fact made from glycerol and long chain fatty acids. Chapter 4 46

47 Name Functional Group Name Functional Group alkene C=C ether alkyne C C amine NH 2 halide F, Cl, Br, or I amide CNH 2 alcohol H carboxylic acid CH aldehyde CH ester Cketone C aromatic ring Chapter 4 47