Chapter 4 Alcohols and Alkyl Halides

1. Chapter 4Alcohols and Alkyl Halides

2. Functional Groups A functional group is a structural unit in a molecule responsible for its characteristic physical properties as well as its behavior under a particular set of reaction conditions.Alkenes and alkynes are examples of functional groups. In this chapter we specifically meet alkylhalides and alcohols

4. Nomenclature IUPAC rules permit the use of two different naming conventions. One is functional class nomenclature the other is substitutive nomenclature.Substitutive nomenclature is preferred.Functional class nomenclature is more common.

5. Functional Class Nomenclature of Alkyl Halides The alkyl group and the halide are listed separately in the name. The alkyl group is the longest chain starting at the carbon that has the halogen attached. Other alkyl groups are listed as substituents.

6. Substitutive Nomenclature of Alkyl Halides Alkyl halides have a halo (fluoro-, chloro-, bromo- and iodo-) substituents on an alkane chain. The halogen is treated as a substituent. The carbon chain is numbered from the side closest the substituent as before.

7. Substitutive Nomenclature of Alkyl Halides Number from side closest to substituent. The halogen and alkyl groups have the same priority. In the name list the substituent alphabetically. 5-chloro-2-methylheptane 2-chloro-5-methylheptane

8. Functional class names have the alkyl name followed by alcohol as a separate word. Substitutive names start with the longest contiguous carbon chain that bears the –OH group. Number from the side closest to the OH group and replace the –ane of the corresponding alkane with –ol. List substituents and their locants before the parent name. IUPAC Nomenclature of Alcohols

9. The OH is assumed to be attached to C-1 of cyclic alcohols. IUPAC Nomenclature of Alcohols 3-ethylcyclopentanol 4-bromobutan-1-ol

10. Classes of Alcohols and Alkyl Halides

11. Alkyl halides are defined as primary if the carbon that the halogen is attached to is directly attached to one other carbon. Similarly if the carbon that the halogen is attached to is directly attached to two carbons then it is a secondary alkyl halide. In tertiary alkyl halides the carbon with the halogen attached is directly attached to three other carbons. Classification of Alkyl Halides secondary tertiary primary

12. Alcohols are defined as primary, secondary or tertiary in the same way. If one, two or three other carbons are directly attached to the carbon that the OH is attached to then the alcohol is primary, secondary or tertiary respectively. Classification of Alcohols secondary tertiary primary

13. Bonding in Alcohols and Alkyl Halides

14. The C-O bond is made by overlap of an sp3 orbital on carbon with one on oxygen. The oxygen has two non bonding electron pairs. Bonding in Alcohols

15. The halogen is connected to the carbon with a s bond. The carbon-halogen bond distances increase in the order: C-F < C-Cl < C-Br < C-I Alkyl halides and alcohols have polar bonds and may be polar molecules. Bonding in Alkyl Halides

16. Physical Properties of Alcohols and AlkylHalides: Intermolecular Forces

17. Molecules with permanent dipoles have a stronger dipole-dipole intermolecular interaction than alkanes. Dipole-dipole Attractive Force Consequently fluoroethane has a higher boiling point than propane despite being almost the same size.Dipole-dipole interactions are not enough to explain the exceptionally high boiling point of ethanol.

18. Alcohols have a special type of dipole-dipole interaction called hydrogen bonding. The partially positive proton of one ⎯ OH group interacts with the partially negatively oxygen of a second ethanol. Alcohols and Hydrogen Bonding The oxygen is termed the hydrogen bond acceptor and the OH hydrogen the hydrogen bond donor.

19. Ball and stick model showing a hydrogen bond between two ethanol molecules. Alcohols and Hydrogen Bonding

20. A space-filling model showing the electrostatic potential for two hydrogen bonded ethanol molecules. Alcohols and Hydrogen Bonding

21. Hydrogen bonds are 15-20 times weaker than covalent bonds. Hydrogen bonding in organic compounds involves O and N only: Hydrogen Bonding Hydrogen bonds are strong enough to impose structural order on many systems.

22. Boiling Points Iodine is highly polarizable because the valence electrons are far from the nucleus. Therefore the induced dipole-induced dipole attractive forces dominate.

23. Boiling Points Increasing the number of halogens (Cl, Br or I) also increases the induced dipole-induced dipole attractive forces and therefore also the boiling point.

24. Boiling Points Fluorine has very low polarizability and the boiling points do not increase with increasing numbers of fluorine atoms.

25. Solubility in Water Alkyl halides are insoluble in water whereas the solubilty of alcohols in water is directly related to the size of the alkyl group the OH is attached to. Methyl, ethyl, n-propyl, and isopropyl alcohols are all totally miscible in water (soluble in all proportions) but only 1 mL 1-octanol dissolves in 2000 mL of water. Hydrogen bonding between ethanol and water.

26. Density Alkyl fluorides and chlorides are less dense, and alkyl bromides and iodides more dense, than water. Increasing halogenation increases density so CH2Cl2 is more dense than water.

27. Preparation of Alkyl Halides fromAlcohols and Hydrogen Halides

28. Preparation of Alkyl Halides • Synthesis. The rest of this chapter focusses on methods of preparation of alkyl halides. • Mechanism.The step-by-step description of how reactions take place will be introduced.

29. Preparation of Alkyl Halides • Reaction of alcohols with hydrogen halides yields alkyl halides: Reactivity of the alcohols is directly related to the nature of the alcohol:

30. Rate of Reaction • Tertiary alcohols react fastest at low temperature and primary slowest needing higher temperatures:

31. Reaction of Alcohols with HydrogenHalides: The SN1 Mechanism

32. The Reaction Equation • The reaction equation describes the overall process from reactants on the left to products on the right. The mechanism will show how this reaction occurs.

33. The Reaction Mechanism: Step 1 • The reaction mechanism is the step-by-step pathway describing how the reaction takes place. • Step 1: Protonation The alcohol acts as Brønsted base and is protonated by the strong acid. Chloride is the conjugate base of HCl.This is a bimolecular reaction – both reactants change.The tert-butyloxonium cation is an intermediate.

34. Proton Transfer • The change in energy of Step 1 can be plotted on a potential energy diagram. The transition state is not a stable structure and the bonds are partially formed and partially broken at this point. The activation energy is low and the step is exothermic.

35. The Reaction Mechanism: Step 2 Step 2: Dissociation. • The second step is unimolecular and results in the formation of an intermediate carbocation.

36. Carbocation Formation • The carbocation intermediate is a relatively unstable species and is therefore high energy (the central carbon does not have an octet of electrons). Overall step 2 is endothermic.

37. Carbocations • The central carbon of the carbocation is sp2 hybridized.The positive charge is in theempty p-orbital. • Carbocations are electrophilic(electron seeking or electronloving) and are Lewis Acids.

38. The Reaction Mechanism: Step 3 • The last step is a Lewis acid-Lewis base reaction. The chloride is called a nucleophile (nucleus seeker). This is a bimolecular reaction. Step 3: Chloride attaches to the carbocation.

39. Reaction of t-Butyl Cation with Chloride • An unstable intermediate reacts to form stable products very exothermic. A very favorable process so the activation energy is low.

40. Reaction of t-Butyl Cation with Chloride • The nucleophile has a nonbonding electron pair in a p-orbital that interacts with the empty p-orbital of the carbocation to form a s-bond.

41. The Reaction Mechanism • The sum of each individual step in the reaction mechanism must equal the overall reaction equation. • The reaction is a substitution reaction in which thenucleophile chloride takes the place of the OH. Thus, it is known as an SN reaction.The slow step in the unimolecular reaction step 2 and this is known as the rate determining step. The overall reaction cannot go faster than this step. Thus, the reaction is a SN1 reaction.

42. Confirming the Mechanism • The stereochemistry of a reaction is used to probe the formation of a carbocation. For example, these two isomeric alcohols should give the same carbocation: Therefore they should form exactly the same product/s as they do – a 4:1 mixture of isomers.

43. Structure, Bonding, and Stability of Carbocations

44. Stability of Cations • Alkyl groups directly attached to the positively charged • carbon stabilize a carbocation. Carbocations are defined as primary, secondary or tertiary depending on how many carbons are directly attached to the cationic carbon.

45. Stability of Cations • The stability of cations can be modeled and the spread of the positive charge (blue/violet color) seen in the electrostatic potential maps. Methyl cation has intense positive charge. The more the charge is delocalized the more stable the cation is.

46. Stability of Cations • A carbocation is stabilized by delocalization • of electrons from s-bonds b to the positively • charged carbon into the empty p-orbital. • The valence bond model shows orbital overlap. • MO theory predicts a bonding orbital with 2 electrons that spans the b s-bond and the positive carbon.

47. Effect of Alcohol Structure on Reaction Rate

48. The Alcohol Carbocation Connection • The rate determining step is: The rate is only proportional to the concentration of the alkyloxonium cation.

49. The Alcohol-Carbocation Connection • The rate of formation of the carbocation is related to the • stability of the carbocation formed. The transition state • is closer in energy to the carbocation so the activation • energy tracks the stability of the carbocation.

50. Reaction of Methyl and Primary Alcohols with Hydrogen Halides: The SN2 Mechanism