Nucleophilic substitution reaction

From Sajun.org

In chemistry, nucleophilic subsitution is a type of chemical reaction in which one nucleophile (electron donor) replaces another as a covalent substituent of some atom. In the examples given here, this is a carbon atom, but this is far from the only possibility. An example of nucleophilic substitution is the hydrolysis of an alkyl bromide, R-Br, under alkaline conditions, where the "attacking" nucleophile is hydroxide ion, OH-: :R-Br + OH- → R-OH + Br- The bromide ion, Br-, is said to be the leaving group. The two main mechanisms of nucleophilic substitution are called SN1 and SN2. S stands for chemical substitution, N stands for nucleophilic, and the number represents the kinetic order of the reaction. ==SN1== The SN1 reaction takes place in two steps, for example #(CH3)3C-Br → (CH3)3C+ + Br- # (CH3)3C+ + OH- → (CH3)3C-OH This gives the overall reaction :(CH3)3C-Br + OH- → (CH3)3C-OH + Br- SN1 tends to be important when the central carbon atom is surrounded by bulky groups, both because such groups interfere sterically with the SN2 reaction (discussed below) and because substituents on the central carbon increase the rate of carbocation formation (the first reaction in the scheme above). The rate of the overall reaction is essentially equal to that of carbocation formation, which does not involve the attacking nucleophile. Thus the overall rate depends on the concentration of the substrate but not on that of the nucleophile, and the kinetic order of the reaction is one: :rate = k[RX] Because the intermediate carbocation, R+, is planar, the central carbon is not a stereocenter, even if it was a stereocenter in the original reactant, so the original configuration at that atom is lost. Nucleophilic attack can occur from either side of the plane, so the product may consist of a mixture of two stereoisomers. In fact, if the central carbon is the only stereocenter in the reaction, the product will be a racemic mixture. ==SN2== In the SN2 reaction, the addition of OH- and the elimination of Br- take place simultaneously. SN2 occurs where the central carbon atom is easily accessible to the nucleophile. The rate is second order, depending on the nucleophile concentration as well as the substrate. :rate = k[RX][OH-] The nucleophile enters on the opposite side of the carbon to the leaving group, so a stereocenter is inverted by an SN2 reaction. 

친핵체의 결합이 형성되는 것과 이탈기의 결합이 끊어지는 것이 동시에 일어나는 한 단계 반응이다. 

① 반응속도는 친핵체와 기질, 두 물질의 농도에 의존한다.
② SN2메커니즘으로 일어나는 모든 치환반응은 배위가 반전되면서 일어난다.
③ 기질의 알킬기가 메틸기 또는 1차일 때 반응속도가 가장 빠르고, 3차일 때 가장 느리다. 2차 알킬 할로젠화물은 중간 정도의 속도로 반응이 진행된다.