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Experiment 9: Nucleophilic Substitution Reactions of Organic Halides
Introduction: Substitution Reactions Substitution reactions are reactions where the two species involved exchange parts:
There are two types of substitution reactions that commonly occur in organic chemical reactions, the S (^) N1 and the S (^) N2 reactions. They have different mechanisms which means their ability to occur is determined by factors like sterics around the alkyl halide, the choice of nucleophile used or even the solvent for the reaction.
1. S (^) N1 (Substitution, Nucleophilic, Unimolecular) Pertinent S (^) N 1 Facts The S (^) N1 reaction is called “unimolecular” because the rate of this type of reaction is dependent upon only one species in the rate-determining step, the alkyl halide.
The rate of reaction is ONLY dependent on how quickly a carbocation might form by “spontaneous dissociation”. Remember that the nucleophile is NOT involved in this step.
Factors that affect the rate of an S (^) N 1 Reaction:
- The Alkyl Halide must form a stable carbocation, and the more stable the carbocation, the faster it will form, causing the overall reaction to occur quicker (more stable intermediate = lower activation barrier).
Recall that alkyl groups stabilize carbocations through both inductive effect and hyperconjugation.
The order of carbocation stability: 3º > 2º >>>>>> 1º or methyl
Resonance Stabilization adds a tremendous amount of stabilization to a carbocation and can also allow an S (^) N 1 reaction to occur faster since resonance also stabilizes carbocations – the more the charge can be spread out over multiple atoms, the more stable the charge will be (“delocalization”). An allylic or Benzylic 1º carbocation is easily as stable as a normal 2º carbocation.
- Leaving Group : Same order of reactivity seen for S (^) N2 reaction Halides: I⊖, Br ⊖^ > Cl ⊖, H 2 O >>> F⊖
(best) (worst)
C X r.d.s.^ C +^ X^ Nuc (slow) (fast)
C Nuc
C C^
Na (^) + Br C C^ + NaBr
- The Nucleophile does NOT affect the rate of the reaction because it is not part of the rate-determining step. The nucleophiles in this type of reaction are often weak bases, usually neutral molecules like H 2 O or ROH. Today we are using ethanol, CH 3 CH 2 OH, as your nucleophile.
- The choice of solvent affects rate by affecting a reagent’s energy levels. Solvent molecules do what is called “solvation”, meaning they surround certain species in a reaction. The solvent can affect the rate of S (^) N 1 reaction by stabilizing the carbocation intermediate as it forms, thus increasing the rate of reaction. Any POLAR solvent can be used to do this. The solvent today is ethanol, CH 3 CH 2 OH, and it will act as both the nucleophile AND the solvent (a “solvolysis” reaction).
S (^) N 1 Reaction Conditions: Each compound will be treated with a solution of silver nitrate in ethanol. Ethanol is a polar, protic solvent that favors ionization of the alkyl halide to form stable carbocations. This is facilitated by the silver ion coordinating to the halide through a lone pair on the halogen atom, causing a weakening of the carbon-halogen bond. Silver halide salts are very insoluble and will precipitate from the solution, indicating if a reaction has occurred. The nitrate ion and ethanol solvent are both poor nucleophiles and so S (^) N2 reactions do not occur.
2. The S (^) N2 Reaction (Substitution, Nucleophilic, Bimolecular) The S (^) N2 type reaction is a one-step, concerted substitution process (make new bonds, break old bonds simultaneously). Both the alkyl halide and the nucleophile are involved (“ bimolecular ”) to determine the rate of reaction:
H O CH 2 CH 3
H
O CH 2 CH 3 H
H 3 CH 2 C O
H H 3 CH 2 C O
H O
CH 2 CH 3
H
O CH 2 CH 3
R X
Ag + R X Ag (^) R + AgX(s)
HOCH 2 CH 3
R-OCH 2 CH 3
Introduction: Experiment In this experiment the reactivity of seven different alkyl halides towards nucleophilic substitution reactions will be examined. There are several factors that may influence the reactivity, including the structure of the substrate, the leaving group and the reaction conditions (both solvent and temperature). Here, we will perform two sets of experiments to test the reactivity of each, under conditions that favor S (^) N2 and under conditions that favor S (^) N1. The structures of the substrates are given below.
FOR YOUR SAFETY
Wear Gloves - Organic halides are irritants and in certain cases corrosive. Silver salts will stain skin black and this is very hard to remove. Wear gloves and dispense with the syringes that are provided.
Procedure A. Reactivity with AgNO 3 in ethanol (S (^) N 1)
- Obtain a thermometer. Add approximately 1 inch depth of hot tap water to a large beaker and place on your hotplate at a setting of 2 to set up a water bath and stabilize the water temperature at 45°C. Clamp a thermometer in the water so that you can monitor the temperature. Make sure the thermometer is not touching the bottom of the beaker (or you’ll be recording the temperature of the glass beaker, not the water!)
- To your clean test tubes labeled #1-7 (washed from part A), and using the color- coded 1.0 mL syringes provided, add 0.1 mL of an organic halide ( in this order, to avoid confusion ):
(1) 1-chlorobutane (2) 2-chlorobutane (3) 2-bromobutane
R Cl + Na +I - acetone R I + NaCl(s)
R Br + Na +I - acetone R I + NaBr(s)
CH 3 CH 2 CH 2 CH 2 Cl CH 3 -CH-CH 2 CH 3 Cl
CH 3 -CH-CH 2 CH 3 Br
Br
1-chlorobutane 2-chlorobutane 2-bromobutane bromobenzene
CH 3 -CH-CH 2 Cl CH 3
H 3 C C
CH 3 Cl CH 3
CH 3 CH=CHCH 2 Cl
1-chloro-2-methylpropane 2-chloro-2-methylpropane 1-chloro-2-butene (cis/trans mixture)
(4) bromobenzene (5) 1-chloro-2-methylpropane (6) 2-chloro-2-methylpropane (7) 1-chloro-2-butene Place a cork stopper in each of the tubes and stand the tubes in a test tube rack.
Again, to avoid confusion during Steps 3-6, do not attempt to do all 7 test tubes in a single trial. Perform Steps 3-6 on the first three compounds listed, then repeat Steps 3-6 on the final four compounds.
- Rapidly transfer 1 mL of a 1% AgNO 3 solution in ethanol to each tube and mix thoroughly by swirling. Replace the cork stopper in each tube and start timing.
Note 1 : the designated timekeeper should start a running timer (Stopwatch on cell phone!) and make note of the START TIME for each addition to each test tube (i.e. 0:02 min, 0:25 min, 0:50 min, etc).
- Watch each test tube carefully and note the time at which a precipitate appears (END TIME) and its color. The timekeeper must record the end time from the running stopwatch (1:03 min, 4:15 min, etc).
Note 2 : The time for the reaction is the difference between the START and the END (0:02 to 1:03 = 1 min, 1 sec and 0:25 to 4:15 = 3 min, 50 sec. Remember we are using min/sec and there are 60 seconds per min!
- If after 5 minutes no precipitate is visible, move any test tubes that have no precipitate into the 45°C water bath and continue watching them for the formation of precipitates.
- Heat for no more than 6 more minutes (11 minutes total), noting the END TIME for any precipitates that form in the hot bath, then remove the tubes from the hot bath and cool to room temperature.
- When your observations are completed , pour all the solutions into the silver salts waste container , rinse the tubes with ethanol and place those washings in the same container. A 10% thiosulfate solution can be used to remove any residual silver halide precipitates (thiosulfate ion dissolves silver halides) – place these washings in the silver waste container also. Then wash with water and rinse with acetone (dispose of acetone rinses in acetone waste). Unless told otherwise, use the same test tubes for part B below.
ion left for as this would give future false positive results for anyone doing the S (^) N1 series of reactions in your test tubes.
DATA TABLES – Set up two tables – one for each type of substitution reaction – and record the following information in your notebook:
Reaction: (Insert type) (^) Room Temperature 45ºC Water Bath (if needed) Compound #: Name Timer Timer Precipitate? Timer Timer Precipitate? Start Stop Y/N (color) Start Stop Y/N (color) 1 : 1-chlorobutane 2 : 2-chlorobutane 3: 2-bromobutane 4: bromobenzene 5: 1-chloro-2-methylpropane 6 : 2-chloro-3-methylpropane 7: 1-chloro-2-butene
