A nucleophilic aromatic substitution is a substitution reaction in organic chemistry in which the nucleophile displaces a good leaving group, such as a halide, on an aromatic ring.
In the gas-phase reactions of halonitro-and dinitrophenide anions with X (X = F, Cl, Br, NO 2) and NO 2 groups in ortho or para position to each other with selected C-H acids: CH 3 CN, CH 3 COCH 3, and CH 3 NO 2, products of the S N Ar-type reaction are formed. In the present paper, we describe the results obtained for the reactions of various halonitrophenide and dinitrophenide anions generated from the respective halonitro-and dinitrobenzoic acids with gas-phase C–H acids like acetone, acetonitrile, and nitromethane. Such addition-elimination processes generally occur at sp 2 or sp hybridized carbon atoms, in contrast to S N 1 and S N 2 reactions. When applied to aromatic halides, as in the present discussion, this mechanism is called SNAr. When a benzene ring has two substituent groups, each exerts an influence on subsequent substitution reactions. The activation or deactivation of the ring can be predicted more or less by the sum of the individual effects of these substituents. The following is the reaction mechanism of a nucleophilic aromatic substitution of 2,4-dinitrochlorobenzene in a basic aqueous solution. In this sequence the carbons are numbered clockwise from 1–6 starting with the 1 carbon at 12 o'clock, which is bonded to the chloride. Aryl halides cannot undergo SN2 reaction .The C–Br bond is in the plane of the ring as the carbon atom is trigonal. To attack from the back, the nucleophile would have to appear inside the benzene ring and invert the carbon atom in an absurd way. This reaction is not possible. S N 2). Nucleophilic substitution is the reaction of an electron pair donor (the nucleophile, Nu) with an electron pair acceptor (the electrophile). An sp 3 -hybridized electrophile must have a leaving group (X) in order for the reaction to take place. Since the nucleophile is free to attack from either side, this reaction is associated with racemization. In both reactions, the nucleophile competes with the leaving group. Because of this, one must realize what properties a leaving group should have, and what constitutes a good nucleophile. An example of a reaction taking place with an S N 1 reaction mechanism is the hydrolysis of tert-butyl bromide with water forming tert-butanol : This S N 1 reaction takes place in three steps: 1  Formation of a tert-butyl carbocation by separation of a leaving group (a bromide anion) from the carbon atom: this step is slow and reversible. 2  Nucleophilic attack: the carbocation reacts with the nucleophile. 1 When the solvent is water, the intermediate is an oxonium ion. 2  This reaction step is fast. 3  Deprotonation: Removal of a proton on the protonated nucleophile by water acting as a base forming the alcohol and a hydronium ion. CHM220 - Studying S N 1 and S N 2 Reactions 2 | P a g e 2. Caron Group: An S N 2 reaction occurs fastest when the approach of the nucleophile to the carbon is unhindered by the presence of bulky groups. For example, the reaction occurs faster at primary carbon atoms than at secondary carbon atoms. 3. CHM220 - Studying S N 1 and S N 2 Reactions 8 | P a g e Questions to answer in your lab report. 1. The limiting reagent in the production of 1-bromobutane is 1-butanol. Calculate the theoretical yeild of 1-bromobutane for your experiment. The benzylic hydrogens of alkyl substituents on a benzene ring are activated toward free radical attack, as noted earlier. Furthermore, S N 1, S N 2 and E1 reactions of benzylic halides, show enhanced reactivity, due to the adjacent aromatic ring. When a benzene ring has two substituent groups, each exerts an influence on subsequent substitution reactions. The activation or deactivation of the ring can be predicted more or less by the sum of the individual effects of these substituents. In the S N 1 reaction, a planar carbenium ion is formed first, which then reacts further with the nucleophile. Since the nucleophile is free to attack from either side, this reaction is associated with racemization. In both reactions, the nucleophile competes with the leaving group. Because of this, one must realize what properties a leaving group should have, and what constitutes a good nucleophile. For this reason, it is worthwhile to know which factors will determine whether a reaction follows an S N 1 or S N 2 pathway A nucleophilic aromatic substitution is a substitution reaction in organic chemistry in which the nucleophile displaces a good leaving group, such as a halide, on an aromatic ring. There are 6 nucleophilic substitution mechanisms encountered with aromatic systems: 1  the SNAr (addition-elimination) mechanism. Aryl halides cannot undergo SN2 reaction .The C–Br bond is in the plane of the ring as the carbon atom is trigonal. To attack from the back, the nucleophile would have to appear inside the benzene ring and invert the carbon atom in an absurd way. This reaction is not possible. Abstract. A wide literature exists on S N Ar reactions of aromatic and heteroaromatic halides with or without activating nitro groups. Most likely the first event is not the formation of a Meisenheimer adduct but an Electron Transfer process leading to radical intermediates. 
http://www.sciencedirect.com/science/article/pii/S1044030507003327 http://www2.chemistry.msu.edu/faculty/reusch/VirtTxtJml/benzrx2.htm https://en.wikipedia.org/wiki/SNAr http://www.organic-chemistry.org/namedreactions/nucleophilic-substitution-sn1-sn2.shtm https://en.wikipedia.org/wiki/SN1_reaction http://classes.kvcc.edu/chm220/nuc%20subst%20reaction-tech714.pdf http://www2.chemistry.msu.edu/faculty/reusch/VirtTxtJml/benzrx2.htm http://www.organic-chemistry.org/namedreactions/nucleophilic-substitution-sn1-sn2.shtm https://en.wikipedia.org/wiki/SNAr http://www.sciencedirect.com/science/article/pii/S0040403900611693
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