Presentation is loading. Please wait.

Presentation is loading. Please wait.

Alkyl Halides Nucleophilic Substitution and Elimination.

Similar presentations


Presentation on theme: "Alkyl Halides Nucleophilic Substitution and Elimination."— Presentation transcript:

1 Alkyl Halides Nucleophilic Substitution and Elimination

2 Nomenclature of Alkyl Halides l Name halogen as substituent on alkane or cylcoalkane. l Learn common names for some of the simple structures. e.g. chloroform, methylene chloride. l Note degree of substitution - name as type of C it is bonded to (i.e. 1 0, 2 0, 3 0 ). l Geminal (gem-) dihalide has two halogen atoms bonded to the same carbon. l Vicinal (vic-) dihalide has two halogens bonded to adjacent carbons. Do problem 6-1, 6-2 and 6-3 of the text.

3 Example Problems:

4 Uses and General Chracteristics l BOND DIPOLE (  ):  + at C,  - at X »all reactions based on this. »The bond dipole moments increase in the order: C—I < C—Br < C—F < C—Cl l Physical properties »generally, trends are similar to those seen in alkanes. »bp affected by London forces and dipole-dipole attractions. l Common uses: solvents, anesthetics, freons (refrigerants), pesticides.

5 Preparation of alkyl & allylic halides l Free radical halogenation of alkanes (Chpt 4) l You are expected to know the mechanism by which this transformation takes place.

6 l Free radical halogenation of alkenes at allylic position l need to know resonance structures for intermediate & predict major/minor product Preparation of Alkyl Halides See pages of the text. Do problems 6-8 and 6-9.

7 Nucleophilic Substitution (S N ) R—LG + Nuc:  R—Nuc + LG: l Substrate l Reagent/Nucleophile (Nuc) l Leaving Group (LG) l Solvent/Reaction Conditions

8 1. Identify electrophilic carbon in substrate 2. Identify nucleophilic electrons in nucleophile 3. Identify leaving group in substrate

9 Then draw product(s) 4. Draw substrate without LG but with bond 5. Add Nuc to bond where LG used to be

10 Factors influencing what products are formed l Substrate/steric effects l Strength of nucleophile vs. basicity of nucleophile l Stability of leaving group l Reaction conditions »Polarity of solvent »acidic/neutral/basic

11 Substitution Mechanisms l Continuum of possible mechanisms l Mechanism determined primarily by substrate steric effects S N 2 - methyl, 1º & unhindered 2º S N 1 - 3º, hindered 2º

12 Bimolecular (S N 2) Nucleophilic Substitution l concerted reaction; Nuc attacks, LG leaves l pentacoordinate carbon in transition state l rate depends on conc. of both reactants l Me = methyl group; Et = ethyl group

13 Reaction is “stereospecific” l 100 % inversion of configuration You should know how to represent this mechanism in an energy diagram!!

14 Factors that Affect S N 2 Reaction Rates l Strength of Nucleophile: species with negative charge is a stronger nuc than an analogous neutral species (e.g. - OH > H 2 O; - NH 2 > NH 3 ). l Nucleophilicity increases from left to right across the periodic chart (e.g. - OH > - F). l Nucleophilicity increased down the periodic table (I - > Br - > Cl - > F - ) or ( - SeH > - SH > - OH). l Solvent: Polar protic solvents (e.g. ethanol, ammonia decrease nucleophilicity. Polar aprotic solvents e.g. (acetonitrile, DMSO, acetone) increase nucleophilicity.

15 l Steric Effects: When bulky groups interfere with a rxn. because of their size, this is called steric hindrance. Steric hindrance affects nucleophilicity, not basicity. (e.g. ethoxide ion is a stronger base than t-butoxide ion). Also, alkyl halide reactivity decreases from methyl to 1 0 to 2 0 to 3 0. In fact, 3 0 alkyl halides do not react by S N 2. l Leaving group: The substrate should have a good leaving group. A good leaving group should be electron withdrawing, relatively stable, and polarizable. They are weak bases. Examples are Cl -, Br -, I -, RSO 3 -, RSO 4 -, RPO 4 -, and neutral molecules such as water, alcohols and amines. Strong bases (OH -, RO -, H 2 N - ) are not good leaving groups! Factors that affect S N 2 reaction rates

16 Unimolecular (S N 1) Nucleophilic Substitution l Two-step reaction »LG leaves, then Nuc: attacks l Tricoordinate carbocation intermediate l Solvolysis (when solvent is also the nucleophile = S N 1 reaction l Rate depends on substrate conc. only

17 Mechanism of S N 1 reaction You must be able to represent this on an energy diagram!

18 Reaction Not Stereoselective Unless Steric Factors Apply l Racemization - not always exactly 50/50. Carbocation can be attacked from the top or bottom face giving both enantiomers. l Steric hindrance gives attack at one side preferentially l Longer-lived carbocations give more racemization, shorter-lived give more inversion

19 Factors Influencing S N 1 Reaction Rates l Stability of the carbocation* Allylic 3° >> 3°  allylic 2° > 2°  allylic 1° >> 1° > Me Carbocations are stabilized by alkyl groups (through hyperconjugation and the inductive effect) and by resonance. l Leaving group stability: the better the leaving group, the faster the reaction. l Solvent polarity: the reaction is favored in polar protic solvents. * must have neutral to acidic conditions to form carbocation

20 Rearrangement of Carbocations l Large difference in energy (stability) of 3° vs. 2° C+ l H - (hydride) or R - will shift (migrate) to adjacent position to form more stable carbocation. E.g. when neopentyl bromide is boiled in methanol, only rearranged product is formed.

21 Elimination Reactions l May proceed by a unimolecular (E1) or bimolecular (E2) mechanism. l In an alkyl halide, when a halide ion leaves with another atom or ion, the reaction is an elimination. l If the halide ion leaves with H +, the reaction is called a dehydrohalogenation.

22 Elimination Mechanisms l Mechanism determined primarily by substrate steric effects

23 E 1 mechanism

24 E1 & S N 1 Competition l Always » by definition a nucleophile is Lewis Base

25 Carbocations generally always give both products l Relative amounts not easily predictable l Always assume formed in approximately equal amounts

26 E2 is Stereospecific l anti-coplanar elimination of H and LG

27 Product Distribution in E2 l Seytzeff Product, most substituted »major with small base, i.e., ethoxide, small LG + major minor R 2 C=CR 2 > R 2 C=CHR > RHC=CHR > R 2 C=CH 2 > RHC=CH 2 Decreasing alkene stability

28 E2 Mechanism l Concerted, anti-coplanar,  Stereospecific l strong base & good LG

29 Elimination is stereospecific

30 Comparison of S N 1 and S N 2 l Base strength unimportant l Substrates: reactivity order is 3 o > 2 o > 1 o l Solvent: good ionizing solvent required l Rate: depends on substrate conc. only l Stereochemistry: no particular geometry required for slow step; Saytzeff rule followed l Rearrangements: very common l Strong bases required l Substrates: reactivity order is 3 o > 2 o > 1 o l Solvent polarity is not so important l Rate: depends on conc. of substrate and base. l Stereochemistry: coplanar arrangement required in transition state; Saytzeff rule followed l Rearrangements: not possible E1 E2


Download ppt "Alkyl Halides Nucleophilic Substitution and Elimination."

Similar presentations


Ads by Google