#4: Alcohols and Alkyl Halides Flashcards
Functional Group
The atom of group in a molecule most responsible for the reaction the compound undergoes under a prescribed set of conditions.
Mechanism
How the structure of the reactant is transformed to that of the product.
Substitutive Nomenclature for Alkyl Halides
Halogens are treated as substituents on the alkane chain.
Name of the halogen is written as a halo (fluoro, chloro, bromo, and iodo).
Name of the parent alkane ends with an -ane.
The parent carbon chain is numbered so that the substituted carbon gets the lowest number.
When the parent carbon chain has both halogen and an alkyl substituent the two are considered equal ranked and the parent chain is numbered so that lower number is given to the substituent nearer the end of the chain.
Substitutive Nomenclature for Alcohols
Identify the longest alkyl chain that has the -OH group.
Number the alkyl chain so that the carbon containing the -OH gets the lowest number.
Name of the alkane chain ends with -ol.
Hydroxyl group (-OH) out ranks alkyl groups and halogens in determining the direction in which the carbon chain is numbered.
Classes of Alcohols and Alkyl Halides
Alcohols and alkyl halides are classified as primary, secondary, or tertiary according to the degree of substitution of the carbon that bears the functional group. Thus, primary alcohols and primary alkyl haldies are compounds of the type RCH2G (where G is the functional group), secondary alcohols and secondary alkyl halides are compounds of the type R2CHG, and tertiary alcohols and tertiary alkyl halides are compounds of the type R3CG.
Bonding in Alcohols and Alkyl Halides
The carbon thatbears the functional group is sp3-hybridized in alcohols and alkyl haldies. THe bond angles at carbon are approximately tetrahedeal, as is the C-O-H angle. A similar orbital bybridization model applies to alkyl halides, with the halogen connected to sp3-hybridized carbon by a sigma bond. Carbon-halogen bond distances in alkyl halides increase in the order C-F (140 pm) < C-Cl (179 pm) < C-Br (197 pm) < C-I (216 pm).
Carbon-oxygen and carbon-halogen bonds are polar covalent bonds, and carbon bears a partial positive charge in alcohols and in alkyl halides. Alcohols and alkyl halides are polar molecules. The dipole moments of methanol and chloromethane are very similar to each other and to water.
Van der Waals Forces
Van der Waals attractive forces between molecules are of three types.
1) Induced-dipole/induced-dipole forces
2) Dipole/induced-dipole forces
3) Dipole-dipole forces
Induced-dipole/induced-dipole forces are the only intermolecular attactive forces available to nonpolar molecules such as alkanes and are important in polar molecules as attractions.
The dipole-dipole attractive force is easiest to visualize. Two molecules of a polar substance experience a mututal attraction between the positively polaized region of one molecule and the negatively polarized region of the other.
As its name implies, the dipole/induced-dipole force combines features of both the other two. A polar region of one molecule alters the electron distribution in a nonpolar region of another in a direction that produces an attractive force between them.
Boiling Point
Van der Waals forces are very important in explaining boiling point of Alcohols and Alkyl Halides. Consider three compounds, alkane propane, alkyl halide fluoroethane, and the alcohol ethanol. Both of the polar compounds, ethanol and fluoroethane, have higher boiling points than the nonpolar one, propane. We attribute this to a combo of dipole/induced dipole and dipole-dipole attractive forces that are present in the liquid states of ethanol and fluoroethane, but absent in propane.
BPs
- Propane (CH3CH2CH3): - 42 C
- Fluoroethane (CH3CH2F): - 32 C
- Ethanol (CH3CH2OH): 78 C
The most striking difference, however, is that despite the similarity in their dipole moments, ethanol has a much higher BP than fluoroethane. This suggests that the attractive forces in ethanol are unusually strong. They’re an example of a special type of dipole-dipole attraction called hydrogen bonding and involve, in this case, the positively polarized proton of the -OH group of one ethanol molecule with the negatively charged polarized oxygen of another. The oxygen of the -OH group of alcohols serve as a hydrogen bond acceptor, while the hydrogen attached to the oxygen serves as a hydrogen bond donor. Having both hydrogen bond acceptor and donor capability in the same molecule creates a strong network among ethanol molecules in the liquid phase.
Among alkyl halides, the boiling point increases with increasing size of the halogen, which means alkyl fluorides have lowest BPs on alkyl halides. But alcohols overall have higher BPs than alkyl halides. Both of them however do have higher BPs than alkanes.
Solubility in Water
Alkyl halides are all insoluble in water, but low-molecular weight alcohols are soluble in water in all proportions. Their ability to particupate in intermolecular hydrogen bonding not only affects the boiling point of alcohols, but also enhances their water solubility.
Higher alcohols become more “hydrocarbon-like” and less water soluble. 1-Octanol, for eample, dissolves to the extent of only 1 mL in 2000 mL of water. As the alkyl chain gets longer, the hydrophobic effect becomes more important, to the point that it, more than hydrogen bonding, governs the solubility of alcohols.
For alcohols…
1-4 Carbons = Water Soluble
5+ Carbons = Not water soluble
Preparation of Alkyl Halides
We’ll begin with the preparation of alkyl halides from alcohols by reaction with hydrogen halides.
R-OH + H-X –> R-X + H-OH
The order of reactivity of the hydrogen halides parallels their acidity: HI > HBr > HCl > > HF. Hydrogen iodide is used infrequently, however, and the reaction of alcohols with hydrogen flouride is not a useful method for the preparation of alkyl fluorides.
Among the various classes of alcohols, tertiary alcohols are observed to be the most reactive and primary alcohols the least reactive. Tertiary alcohols are converted to alkyl chlorides in high yield within minutes on reaction with hydrogen chloride at room temperature and below. Secondary and primary alcohols don’t react with HCl at rates fast enough to make the preparation of the corresponding alkyl chlorides a method of practical value. Therefore, the more reactive hydrogen halide HBr is used; even then, elevated temperatures are required to increase the rate of reaction.
Mechanism of the Reaction of Alcohols with Hydrogen Halides: Hammond’s Postulate
This reaction is a substitution. A halogen, usually chlorine or bromine, replaces a hydroxyl group as a substituent on carbon. Calling the reaction a substitution tells us the relationship between the organic reactant and product but does not reveal the mechanism. The mechanism is the step-by-step pathway of bond cleavage and bond formation that leads from reactants to products. In developing a mechanistic picture for a particular reaction, we combine some basic principles of chemical reactivity with experimental observations to deduce the most likely sequence of steps.
Consider the reaction of tert-butyl alcohol with hydrogen chloride:
(CH3)3COH + HCl –> (CH3)3CCl + H2O
The mechanism for this reaction is presented as a series of three equations. There is no way to truly prove this reaction is correct though. It can always be modified upon the findings of new data.
Each equation in the mechanism represents a single elementary step, which is one that involves only one transition state. A particular reaction might proceed by way of a single elementary step, in which it is described as a concerted reaction, or by a series of elementary steps.
Mechanism of the Reaction of Alcohols with Hydrogen Halides Step 1: Proton Transfer
ALcoholds resemble water in respect to their Bronsted acidity. They also resemble water in their Bronsted basicity. Just as proton transfer to a water molecule gives oxonium ion (H3O+), proton transfer to an alcohol gives an alkyloxonium ion (ROH2+).
(CH3)3C-O-H + H-Cl (CH3)3C-O-H2+ + Cl-
Furthermore, a strong acid such as HCl that ionizes completely when dissolved in water also ionizes completely when dissolved in an alcohol. Many important reactions of alcohols involve strong acids either as reactants or as catalysts. In all these reactions the first step is formation of an alkyloxonium ion by proton transfer from the acid to the alcohol.
For molecularity, the transfer of a proton from HCl to tert-butyl alcohol is bimolecular because two molecules [HCl and (CH3)3COH) undergo chemical change.
The tery-butyloxonium ion [(CH3)3C-O-H2+] formed in step 1 is an intermediate. It’s not one of the initial reactants, nor is it formed as one of the final products. Rather it’s formed in one elementary step, consumed in another, and lies ont he pathway from reactants to products.
Molecularity
The molecularity of an elementary step is given by the number of species that undergo a chemical change in that step.
Mechanism of the Reaction of Alcohols with Hydrogen Halides Step 1: Proton Transfer (Potential Energy Diagram)
Shown on page 150. These aspects worth noting.
- The point of max potential energy encountered by the reactants as they proceed to products is called the transition state.
- The difference in energy between the reactants and transition state is known as the energy of activation Eact.
- Because this is an elementary step, it involves a single transition state.
- This step is known to be exothermic, so the products are placed lower in energy than the reactants. It is exothermic because HCl is a stronger acid than the alkyloxonium ion.
- Proton transfers from strong acids to water and alcohols rank among the most rapid chemical processes and occur almost as fast as the molecules collide with one another. Thus the height of the energy barrier, the Eact for proton transfer, must be quite low.
The concerted nature of proton transfer contributes to its rapid rate. The energy cost of breaking the H-Cl bond is partially offset by the energy offset by the energy released in forming the new bond between the transferred proton and the oxygen of the alcohol. Thus, the activation energy is far less than it would be for a hypothetical two-step process in which the H-Cl bond breaks first, followed by bond formation between H+ and the alcohol.
The species present at the transition state is not a stable structure and cannot be isolated or examined directly. In general, the bonds in transition states are partially rather than fully formed. Its structure is assumed to be one in which the proton being transferred is partially bonded to both chlorine and oxygen simultaneously, although not necessarily to the same extent.
Hammond’s Postulate
If two states are similar in energy, they are similar in structure.
One of its corollaries is that the structure of a transition state more closely resembles the immediately preceding or following state to which it is closer in energy.
