Organic Chemistry

1. Organic Chemistry Organic chemistry is the study of carbon compounds, excluding CO, CO2, CS2 and various carbonates, bicarbonates, and cyanides, which are traditionally considered to be inorganic compounds. Today, well over 13 million synthetic and natural organic compounds are known. This number is significantly greater than the 100,000 or so known inorganic compounds. Organic 2009-2010

2. Carbon can form more compounds than any other element because carbon atoms are able not only to form single, double, and triple carbon-carbon bonds, but also to link up with each other in chains and ring structures. Organic 2009-2010

3. Classes of organic compounds All organic compounds are derived from a group of compounds known as hydrocarbons because they are made up of only hydrogen and carbon. Aromatic hydrocarbons contain one or more rings with delocalized electrons (commonly benzene). Aliphatic hydrocarbons do not contain rings with delocalized electrons. Organic 2009-2010

4. Aliphatic hydrocarbons are divided into alkanes, alkenes and alkynes. Organic 2009-2010

5. Alkanes • Alkanes have the general formula CnH2n+2, where n = 1,2,3,… • The essential characteristic of alkane hydrocarbon molecules is that only single covalent bonds between the carbons are present. • The alkanes are known as saturated hydrocarbons because they contain the maximum number of hydrogen atoms that can bond with the number of carbon atoms present (there are no multiple bonds between carbons). Organic 2009-2010

6. Alkane nomenclature • The nomenclature of alkanes and all other organic compounds is based on the recommendations of the International Union of Pure and Applied Chemistry (IUPAC). Organic 2009-2010

7. The parent name of the hydrocarbon is that given to the longest continuous chain of carbon atoms in the molecule. Organic 2009-2010

8. There are several different ways to represent an alkane… Expanded formula Condensed formula Carbon skeleton model Ball and stick model Organic 2009-2010

9. Both ball and stick models and Newman projections are useful when an understanding of the spatial relationship is needed Organic 2009-2010

10. Any chain branching off the longest chain is named as an alkyl group, an alkane which has one less hydrogen atom. • For example, when a hydrogen atom is removed from methane, we are left with CH3-, which is called a methyl group. What would a CH3 – CH2- group be called? ethyl Organic 2009-2010

11. CH3 C H CH3 CH3 C CH3 CH3

12. Explain why 4-methyl pentane is not the correct name for this structure. 5 4 3 2 1 1 2 3 4 5 When one or more hydrogen atoms are replaced by other groups, the name of the compound must indicate the locations of carbon atoms where replacements are made. The procedure is to number each carbon atom on the longest chain in the direction that gives the smaller numbers for the locations of all branches. Because numbering the carbon chain from the other end would have given a smaller number to the methyl group, therefore the correct name for this structure is 2-methyl pentane. Organic 2009-2010

13. When there is more than one alkyl branch of the same kind present, we use a prefix such as di-, tri-, or tetra- with the name of the alkyl group. CH3 CH CH CH2 CH2 CH3 CH3 CH3 This compound’s name would be 2,3-dimethyl hexane Organic 2009-2010

14. What would the name of this compound be? 3,3-dimethyl pentane Organic 2009-2010

15. CH3 CH CH CH2 CH2 CH2 CH3 CH2CH3 • When there are two or more different alkyl branches, the name of each branch, with its position number, precedes the parent name. • When two or more different branches occur, their names appear in alphabetical order. For example, CH3 would be… 3-ethyl-2-methylheptane Organic 2009-2010

16. Names of Common Substituent Groups Of course, alkanes can have many different types of substituents. Organic 2009-2010

17. CH3 CH CH CH3 Br NO2 What is the name for the following compound? 2-bromo-3-nitrobutane Organic 2009-2010

18. CH3 CH3 CH CH3 CH2 CH2 CH3 CH3 Structural Isomers – Compounds containing the same atoms, but arranged in completely different ways. Draw two structural isomers of C4H10. Both of these compounds share the same chemical formula, C4H10, but being structural isomers, they differ in their physical and chemical properties. Organic 2009-2010

19. Reactions of Alkanes Alkanes are generally not considered to be very reactive substances. However, under suitable conditions they do react. Combustion • Methane and other hydrocarbons undergo highly exothermic combustion reactions. Write a chemical equation showing the complete combustion of gaseous methane, a process which results in the release of 890.4 kJ of energy. CH4(g) + 2O2(g)  CO2(g) + 2H2O(l) DH = -890.4 kJ Organic 2009-2010

20. 100 oC methyl chloride CH4(g) + Cl2(g) CH3Cl(g) + HCl(g) Halogenation Halogenation – is the replacement (substitution) of one or more hydrogen atoms by halogen atoms When a gaseous mixture of methane and chlorine is heated above 100 oC or irradiated with light of a suitable wavelength, methyl chloride and hydrogen chloride are produced. Write a chemical equation showing this reaction. Alkanes in which one or more hydrogen atoms have been replaced by a halogen atom are called alkyl halides. Among the large number of alkyl halides, the best known are chloroform (CHCl3), carbon tetrachloride(CCl4), methylene chloride (CH2Cl2), and the chlorofluorohydrocarbons. Organic 2009-2010

21. Optical Isomerism of Substituted Alkanes Optical isomers are compounds that are nonsuperimposable mirror images. (“Superimposable” means that if one structure is laid over the other, the positions of all the atoms will match.) The structural relationship between two optical isomers is analogous to the relationship between your left and right hands. If you place your left hand in front of a mirror, the image you see will look like your right hand. We say that your left hand and right hand are mirror images of each other, and are nonsuperimposable. Organic 2009-2010

22. Optical isomers are described as chiral (from the Greek word for “hand”) because like your left and right hands, chiral molecules are nonsuperimposable. Nonsuperimposable mirror images of a chiral compound are called enantiomers; enantiomers come in pairs. Molecules that are superimposable are said to be achiral. Organic 2009-2010

23. Optical isomers have identical physical and chemical properties, such as melting point, boiling point and chemical reactivity. Optical isomers DO differ from each other in their interactions with plane-polarized light. Organic 2009-2010

24. Optical isomers rotate a plane of polarized light either to the right or to the left. If the plane of polarization is rotated to the right, the isomer is said to be dextrorotatory (d) or (+) ; it is levorotatory (l) or (-) if the rotation is to the left. Glucose rotates polarized light to the right so it’s also known as dextrose. The d and l isomers of a chiral substance always rotate light by the same amount, but in opposite directions. Thus, in equimolar mixtures of the two enantiomers, called a racemic mixture, the net rotation is zero FYI - Chemists can actually identify the sugar in a solution by which direction and how much it rotates the light. Wine makers routinely use a specially calibrated polarimeter to measure the sugar remaining as the wine ages. Organic 2009-2010

25. Is CH2ClBr a chiral or achiral molecule? Explain. It is an achiral molecule because it can be superimposed on its mirror image It is a chiral molecule because it is nonsuperimposable with its mirror image. Is CHFClBr a chiral or achiral molecule? Explain. Organic 2009-2010

26. Chirality plays an important role in biological systems. Protein molecules have many asymmetric carbon atoms and their functions are often influenced by chirality. Because enantiomers behave differently in the body, they are of great interest to the pharmaceutical manufacturers. Many prescribed drugs are chiral. In most of these drugs, only one enantiomer of the drug works as medicine, whereas the other form is useless or less effective or may even cause serious side effects. The best-known case in which the use of a racemic mixture of a drug had tragic consequences occurred in Europe in the late 1950s. The drug thalidomide had to be withdrawn from the market because although the (+) form has sedative properties, (-) thalidomide is a powerful mutagen. Ibuprofen is also sold as a racemic mixture; the (-) form is a potent pain reliever, the (+) form is ineffective but also harmless. Organic 2009-2010

27. Cycloalkanes Alkanes whose carbon atoms are joined in rings are known as cycloalkanes. Cycloalkanes have the general formula CnH2n, where n = 3, 4, … The simplest cycloalkane is cyclopropane. What is the chemical formula for cyclopropane? C3H6 Organic 2009-2010

28. H H H C H C C H H C H H Draw the correct structure for cyclobutane. Organic 2009-2010

29. Elimination reactions occur when a saturated reactant becomes an unsaturated product. C C C C C C + HBr Br Note the elimination of HBr Organic 2009-2010

30. Alkenes The alkenes (also called olefins) contain at least one carbon-carbon double bond, C=C. Alkenes have the general formula CnH2n, where n = 2, 3, … The simplest alkene is C2H4, commonly referred to as ethylene. Organic 2009-2010

31. Alkene Nomenclature In naming alkenes we indicate the positions of the carbon-carbon double bonds. The names of compounds containing C=C bonds end with –ene. The name of the parent compound is determined by the number of carbon atoms in the longest chain that contains the double bond. Organic 2009-2010

32. CH2=CH-CH2-CH3 would be 1-butene Note that the number in the name of the alkene refers to the lowest numbered carbon atom in the chain that is part of the C=C bond of the alkene. (If the double bond is between carbon 1 and carbon 2, you call it 1-butene, not 2-butene.) What would be the correct name for this compound? CH3-CH=CH-CH2-CH3 2-pentene Organic 2009-2010

33. Alkene nomenclature must also specify whether a given molecule is cis or trans if it is a geometric isomer. Geometric isomers are compounds that are made up of the same types and numbers of atoms bonded together in the same sequence but with different spatial arrangements. The cis isomer has two particular atoms (or groups of atoms) on the same side of the double bond, and the trans isomer means that two atoms, (or groups of atoms) are on opposite sides of the double bond. Organic 2009-2010

34. Cis and trans isomers, like other geometric isomers, generally have quite different colors, melting points and chemical reactivities. 4-methyl-cis-2-hexene 4-methyl-trans-2-hexene Organic 2009-2010

35. Properties and Reactions of Alkenes Alkenes are classified as unsaturated hydrocarbons, compounds with double or triple carbon-carbon bonds that enable them to add hydrogen atoms. (Compounds with more than one multiple bond are considered polyunsaturated.) Unsaturated hydrocarbons commonly undergo addition reactions, in which one molecule adds to another to form a single product. Organic 2009-2010

36. Hydrogenation Hydrogenation of an alkene is an example of an addition reaction. C2H4 + H2 CH3-CH3 Other addition reactions to the C=C bond include C2H4 + HX  CH3-CH2X C2H4 + X2  CH2X-CH2X Where X represents a halogen. Organic 2009-2010

37. C = C + Br2 Br – C – C – Br Br2(aq) Reddish-brown colorless colorless When ethylene gas is bubbled through the aqueous bromine solution, the reddish-brown color gradually disappears due to the formation of 1,2 dibromoethane, which is colorless. This makes bromine an indicator for the presence of an alkene. Organic 2009-2010

38. The addition of a hydrogen halide to an unsymmetrical alkene such as propylene is more complicated because two products are possible: In reality, however, only 2-bromopropane is formed. This phenomena was observed in all addition reactions with alkenes. Markovnikov’s rule states that when adding polar reagents to alkenes, the hydrogen will add to the carbon that already has the most hydrogen atoms bonded to itself. Organic 2009-2010

39. Alkynes Alkynes contain at least one carbon-carbon triple bond, C=C. Alkynes have the general formula CnH2n-2 where n = 2, 3, … Organic 2009-2010

40. Alkyne Nomenclature Names of compounds containing C C bonds end with –yne. Again the name of the parent compound is determined by the number of carbon atoms in the longest chain (containing the triple bond). As in the case of alkenes, the names of alkynes indicate the position of the carbon-carbon triple bond, as, for example, in HC C-CH2-CH3 or CH3-C C-CH3 1-butyne 2-butyne Organic 2009-2010

41. Properties and Reactions of Alkynes The simples alkyne is ethyne, better known as acetylene (C2H2). Acetylene, an unsaturated hydrocarbon, can be hydrogenated to yield ethene: C2H2 + H2 C2H4 Ethyne undergoes the following addition reactions with hydrogen halides and halogens: C2H2 + HX  CH2 = CHX C2H2 + X2 CHX = CHX C2H2 + 2X2 CHX2 – CHX2 Organic 2009-2010

42. Alkynes follow Markovnikov’s rule during addition reactions. What product is formed when hydrogen bromide is added to 1-propyne? 2 - bromopropene You don’t need to state where the double bond is because in propene, the double bond must be between carbon 1 and carbon 2. Organic 2009-2010

43. Aromatic Hydrocarbons Benzene is the parent compound of this large family of organic substances. Benzene is best represented by both of resonance structures shown below. Organic 2009-2010

44. Alternatively, the properties of benzene can be explained in terms of delocalized molecular orbitals. Electrons in delocalized molecular orbitals are not confined to the two adjacent atoms, instead the electrons are free to move around the benzene ring. For this reason, the structure of benzene is commonly represented as a hexagon, (the points of which represent the carbon atoms) containing a circle (which represents the freely moving electrons). The hydrogen atoms are not represented in this model – but we have to remember they are still there! Organic 2009-2010

45. Phenol Toluene Nitrobenzene Aminobenzene (aniline) Chlororbenzene Ethylbenzene Nomenclature of Aromatic Compounds The naming of monosubstituted benzenes, that is, benzenes in which one H atom has been replaced by another atom or group of atoms, is quite straightforward, as shown below: CH2CH3 OH CH3 NO2 NH2 Cl Organic 2009-2010

46. If more than one substituent is present, we must indicate the location of the second group relative to the first. The systematic way to accomplish this is to number the carbon atoms as follows: Numbering can start at any carbon, and can go clockwise or counter clockwise – depending on the location of the substituted groups. Organic 2009-2010

47. The prefixes o- (ortho-), m- (meta-), and p- (para-) are also used to denote the relative positions of the two substituted groups, as shown below for the dibromobenzenes. 1,2 dibromobenzene (o-dibromobenzene) 1,3 dibromobenzene (m-dibromobenzene) 1,4 dibromobenzene (p-dibromobenzene) Organic 2009-2010

48. A group containing benzene minus a hydrogen atom (C6H5) is called the phenyl group. A phenyl can be a branch on a hydrocarbon chain. Thus the following compound is called 2 - phenylpropane Organic 2009-2010

49. Properties and Reactions of Aromatic Compounds Benzene is relatively inert, primarily because of electron delocalization. Benzene can be hydrogenated, but only with difficulty. Organic 2009-2010

50. The most common reaction of halogens with benzene is the substitution reaction, in which an atom or group of atoms replaces an atom or group of atoms on another molecule. Organic 2009-2010