Chart for Deciding Mechanism for Reaction of Alkyl Halide with Nucleophile/Base Bruce W. McClelland Central Oregon Community College, 2600 NW College Way, Bend, OR 97701-5998 In the introductorv " orpanic - chemistrv course, the possihle reactions that can occur between a h alkyl halide R-X and a nucleo~hilelbaseNu are often presented to demonstrate the reiationship between stru&ure and reactivity. The facts that four different reaction mechanisms ( S N ~ , SN2, E l and E2) must he considered and t h a t several might occur simultaneously (depending on the specific alkyl halide and nucleophilehase) make i t difficult for beginning students to sort out what will happen when a specific alkyl halide and nucleophilehase are brought together. Some years ago, as I tried to organize my own thinking on the topic, I began playing with a 'decision chart" that could serve as a guide to the proper choice of mechanism(s) appropriate for a specific set of reactants R-X and Nu. When I presented a crude chart to my nonchemistry major organic students, they received i t enthusiastically and encouraged me to refine and expand it. Over the years, in fact, several students have made suggestions for improvement. The most recent version, used for several years now, is shown in the figure. The decision chart of the figure is based upon the wellknown and acceoted characteristics of the reaction system and mechanisms described i n typical introductory organic chemistrv textbooks. These characteristics, here !ziven approximaielyin the order used in the chart, &ay besummarized as follows. 1. Relative reaction rates determine which mechanism(s1 will be important. 2. Vinylic and aryl halides are generally unreaetive with even a strong nueleophileihase. 3. A strona nucleophile is needed for the SN2mechanism to occur 4. The SN2rate depends on the alkyl halide structure in the order methyl > primary > secondary > tertiary. 5. SNland E l rates depend on alkyl halide structure in the order tertiary > secondary > primary > methyl. 6. Even when a strong nucleophile is present, the SN2rate far a tertiary alkyl halide generally is slaw compared to the SN1rate. 7. Astrong base is needed for the E2 mechanism to occur. 8. The E2 rate depends on alkyl halide structure in the order tertiary > secondary > primary. 9. When a strong base is present, E2 is faster than S Nand ~ El. 10. When Nu is both a strong base and a strong nucleophile, the E2 mechanism is faster for a tertiary alkyl halide and the SN2is faster for a primary alkyl halide. 11 The SN1mechanism goes faster than E l when the solvent is used as a weak nucleophile. Two other considerations have been built into the chart. First, while all mechanisms may occur to somcextcnt, only the domin:tting mechanism is derived from the chart, and two competin&mechanisms are given only if each leads to a very significant amount (nearer 50% a s opposed to nearer 10%)of the products formed. Second, it is assumed that if the fastest mechanism is quite slow, then for practical purposes no reaction occurs. In using the chart, students need to classify a n alkyl halide a s vinylic, aryl, methyl, primary, secondary, or tertiary, a straightforward process. They also need to classify a nucleophile/base a s weak or strong i n each property.
Basicity is determined by looking a t the value of& for the species. If Kb is larger than, say, 1x a species might he called a strong base, otherwise i t would he called a weak base. Thus, ammonia, and alkoxide, hydroxide, and cyanide ions would he classified a s strong bases, whereas carboxvlate anions. alcohols. water. and halide ions would be c1a;sified as weak bases. he n~bleo~hilicity of a species is more difficult to ascertain. although there are well known guiding principles t h a t can he considered, including the following: 1. a negatively charged nueleophile is always stronger than its conjugate acid (e.g. OH- > H20 and RO- > ROH); 2. in a group of nucleophiles in which the nucleaphilie atom is the same element, nucleaphilicity parallels basicity (e.g.R O > O H > RCO< ROH H20);and 3. steric bulk can have a profound effect on nucleophilicity, so that t-butaxide is a weak nucleophile while less bulky alkoxides are strong nucleophiles.
Begin
1
- jR=vmyloran/ll -
No reaction
1
Is Nu a weak base nucieophiie?
primary
I
S,Z
1
Both.%I8
I
S,1
~
Is NUa strong base nucleophile? rehion (except S,2 for Nu like 1-butoxide)
E2
NO
high temp.
is Nu astrong base nudeaphile?
I SN2
no (no Lase or
nucleophile)
1
no
reaction
I
Both
I
E2
S,,28E2
1
%I , I high
I %I
temp
No
El
reaction Decision chart for choice of mechanism for reaction R-X Products Volume 71
Number 12 December 1994
+ Nu + 1047
Ultimately, I tell students that CN-, I; Br-, CT, OH-, RO(R # t-butvl). . . and NHs are strong- nucleophiles, while HzO, ROH, t-butoxide, and RCOpare weak, in common reaction solvents like water or alcohol. chart, the well-known effects To avoid a more of nonreacting solvents on reaction rates have been ignored. However, I discuss these effects with my students, and emphasize them when the focus is on how to maximize the rate of a particular mechanism.
1048
Journal of Chemical Education
Use of this chart can be varied to suit a variety of educational perspectives. Some instructors may prefer to present the chart at the beginning of this topic to serve as an outline for discussion. Others may choose to present it after the discussion as a means of topic summarization. To avoid making my students feel that they must memorize I allow them to use it during exams, hut recluire the that justifies iheir that they' provide a detailed choice(s1 as suggested to them by the chart. I would be interested in finding out how others may use the chart.
