Chapter 4 Alcohols and Alkyl Halides Functional Groups

Chapter 4 Alcohols and Alkyl Halides

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

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.

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.

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.

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. 2 -chloro-5 -methylheptane 5 -chloro-2 -methylheptane

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

Classes of Alcohols and Alkyl Halides

Classification of Alkyl Halides 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. secondary tertiary primary

Classification of 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. tertiary secondary primary

Bonding in Alcohols and Alkyl Halides

Bonding in Alcohols The C-O bond is made by overlap of an sp 3 orbital on carbon with one on oxygen. The oxygen has two non bonding electron pairs.

Bonding in Alkyl Halides 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.

Physical Properties of Alcohols and Alkyl Halides: Intermolecular Forces

Dipole-dipole Attractive Force Molecules with permanent dipoles have a stronger dipole intermolecular interaction than alkanes. 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.

Alcohols and Hydrogen Bonding 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. The oxygen is termed the hydrogen bond acceptor and the OH hydrogen the hydrogen bond donor.

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

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

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

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

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.

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

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, n-propyl, and isopropyl alcohols are all totally miscible in water (soluble in all proportions) but only 1 m. L 1 -octanol dissolves in 2000 m. L of water. Hydrogen bonding between ethanol and water.

Density Alkyl fluorides and chlorides are less dense, and alkyl bromides and iodides more dense, than water. Increasing halogenation increases density so CH 2 Cl 2 is more dense than water.

Preparation of Alkyl Halides from Alcohols and Hydrogen Halides

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.

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:

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

Reaction of Alcohols with Hydrogen Halides: The SN 1 Mechanism

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

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.

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.

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

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.

Carbocations The central carbon of the carbocation is sp 2 hybridized. The positive charge is in the empty p-orbital. Carbocations are electrophilic (electron seeking or electron loving) and are Lewis Acids.

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.

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.

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.

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 the nucleophile 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 SN 1 reaction.

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.

Structure, Bonding, and Stability of Carbocations

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.

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.

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.

Effect of Alcohol Structure on Reaction Rate

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

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.

Reaction of Methyl and Primary Alcohols with Hydrogen Halides: The SN 2 Mechanism

Reaction Equation The overall reaction equation looks the same as that for tertiary alcohols: a substitution reaction.

Mechanism Step 1 The first step of the reaction is the same. Step 1: Proton transfer. The second step of the reaction cannot be the same because the primary carbocation is too unstable

Mechanism Step 2 Since primary and methyl carbocations are very unstable primary alcohols must react in a different way. Step 2. Nucleophilic displacement. Step 2 is the slower step and therefore is the rate determining step. Since this is a bimolecular reaction it is given the symbol SN 2.

Other Methods for Converting Alcohols to Alkyl Halides

Reaction of Alcohols with Thionyl Chloride Thionyl chloride is mainly used to transform primary and secondary alcohols into alkyl chlorides. Pyridine is a base used to neutralize the acid (HCl) formed.

The Thionyl Chloride Reaction Mechanism The key step in the reaction is an SN 2 type of reaction:

Reaction of Alcohols with Phosphorous Tribromide Phosphorous tribromide (PBr 3) reacts with alcohols to form alkyl bromides. This is also an SN 2 reaction.

Halogenation of Alkanes Overall reaction equation: RH + X 2 RX + HX (X=F, Cl, Br or I) For F 2 the reaction is explosive For I 2 the reaction is endothermic and not feasible The reaction is exothermic and useful for Cl 2 and Br 2

Chlorination of Methane An industrially important reaction carried out at high temperature (400 o. C). Four products formed sequentially. CH 4 + Cl 2 CH 3 Cl + HCl CH 3 Cl + Cl 2 CH 2 Cl 2 + HCl CH 2 Cl 2 + Cl 2 CHCl 3 + HCl CHCl 3 + Cl 2 CCl 4 + HCl The reaction has free radicals as intermediates.

Structure and Stability of Free Radicals

Free Radicals Free radicals contain unpaired electrons. Some common free radicals are:

Free Radicals Free radicals with the unpaired electron on a carbon are defined as primary, secondary or tertiary depending on how many carbons are directly attached to the C with the unpaired electron.

Free Radicals The carbon atom with the unpaired electron is best described as sp 2 hybridized.

Stability of Free Radicals The spin density (shown in yellow) is shared onto connected alkyl groups thereby stabilizing the radical. More substituted radicals are more stable.

Stability of Free Radicals More substituted radicals are more stable.

Bond Cleavage In homolytic bond cleavage each atom retains one of the bonding electrons. The energy required is the bond dissociation enthalpy (D). In heterolytic cleavage the more electronegative element retains both bonding electrons. homolytic heterolytic

Bond Dissociation Enthalpies Comparing the energy required for homolytic bond cleavage provides a way to quantify radical stability. Compare these two reactions: The only difference is the type of radical formed and the energy released.

Bond Dissociation Enthalpies Potential energy graph of the two homolytic cleavage reactions. This provides information on the stability of radicals. Less energy: more stable radical.

Free Radical Chlorination of Methane We will consider monochlorination. The mechanism has three parts: initiation, propagation, and termination.

Chlorination of Methane: Initiation Step 1: Dissociation. The chlorine-chlorine bond is broken homolytically. This bond is the weakest in the reaction mixture. Heat or light can provide the necessary energy. Arrows with a single “hook” are used to show the movement of a single electron.

Chlorination of Methane: Propagation Step 2: Hydrogen atom abstraction. A chlorine radical abstracts a hydrogen atom from a methane molecule forming HCl and a methyl radical. The methyl radical reacts in the next step (it propagates the reaction).

Chlorination of Methane: Propagation The methyl radical abstracts a chlorine atom from a chlorine molecule to form chloromethane and another chlorine radical. The chlorine radical starts another propagation cycle – step 2.

Chlorination of Methane: Termination When two radicals react they effectively stop the propagation steps and are therefore known as chain-termination steps. Examples are:

Halogenation of Higher Alkanes Reaction in labs use light (hu) to initiate the reaction. Cyclobutane yields a single monochlorination product since abstraction of any of the 8 H atoms results in the same product being formed. In contrast, butane yields a 28: 72 mixture of 1 chlorobutane to 2 -chlorobutane.

Selectivity of Halogenation There are 6 primary hydrogens and 4 secondary hydrogens in butane. Therefore the expected ratio is 60: 40. The 28: 72 ratio therefore means that 2 -chlorobutane is preferentially formed. Why?

Selectivity of Halogenation There must be selectivity when the hydrogen atom is abstracted. The preferred reaction is: instead of:

Selectivity of Halogenation The transition state is lower in energy for abstraction of a secondary hydrogen because a secondary radical is more stable: The relative rates of hydrogen abstraction are:

Selectivity of Chlorination Based on these experiments the relative rates of chlorination are determined:

Selectivity of Bromination The relative rates of bromination are : Bromination is highly selective favoring tertiary substitution:

Chlorination vs Bromination The hydrogen atom abstraction is exothermic for chlorination and endothermic and therefore more selective for bromination: