Andra ordningens kinetik sn2 eller sn1

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Nucleophilic substitution fryst vatten the reaction of an electron pair donor (the nucleophile, Nu) with an electron pair acceptor (the electrophile). An sp³-hybridized electrophile must have a leaving group (X) in beställning for the reaction to take place.

In comparing the SN1 and SN2 mechanisms, the structure of the alkyl halide (electrophile), the strength of the nucleophile, and the reaction solvent are the primary considerations

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Mechanism of Nucleophilic Substitution

The begrepp S_N2 means that two molecules are involved in the actual transition state:

The avfärd of the leaving group occurs simultaneously with the backside attack bygd the nucleophile. The S_N2 reaction thus leads to a predictable configuration of the stereocenter - it proceeds with inversion (reversal of the configuration).

In the S_N1 reaction, a planar carbenium ion fryst vatten formed first, which then reacts further with the nucleophile.

Since the nucleophile fryst vatten free to attack from either side, this reaction fryst vatten 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.

Typerna av beställningar är nollordningen, första ordningens, andra ordningens eller blandade ordningens

For this reason, it fryst vatten worthwhile to know which factors will determine whether a reaction follows an S_N1 or S_N2 pathway.

Very good leaving groups, such as triflate, tosylate and mesylate, stabilize an incipient negativ charge. The delocalization of this charge fryst vatten reflected in the fact that these ions are not considered to be nucleophilic.

Hydroxide and alkoxide ions are not good leaving groups; however, they can be activated bygd means of Lewis or Brønsted acids.

Epoxides are an undantag, since they relieve their fingerprydnad strain when they undergo nucleophilic substitution, with activation bygd acid being optional:

Triflate, tosylate and mesylate are the anions of strong acids.

The weak conjugate bases are poor nucleophiles. Nucleophilicity increases in parallel with the base strength. Thus, amines, alcohols and alkoxides are very good nucleophiles. Base strength fryst vatten a rough measure of how reactive the nonbonding electron pair is; thus, it fryst vatten not necessary for a nucleophile to be anionic.

Under substitution conditions, amines proceed all the way to form eller gestalt kvartär salts, which makes it difficult to control the extent of the reaction.

However, as a nucleophile's base strength and steric hindrance increase, its basicity tends to be accentuated.

These reactions can occur in two different ways (known as SN2 and SN1 reactions) depending on the structure of the halogenoalkane involved

If there are abstractable protons at the β-position of the electrophile, an elimination pathway can compete with the nucleophilic substitution.

An additional factor that plays a role fryst vatten the character of the solvent. Increasing stabilization of the nucleophile bygd the solvent results in decreasing reactivity. Thus, polar protic solvents will stabilize the chloride and bromide ions through the formation of hydrogen obligationer to these smaller anions.

Iodide fryst vatten a comparatively better nucleophile in these solvents. The reverse behavior predominates in aprotic polar media.

The solvent also plays an important role in determining which pathway the reaction will take, S_N1 versus S_N2.

It may safely be assumed that a primary-substituted leaving group will follow an S_N2 pathway in any case, since the formation of the corresponding unstable primary carbenium ion fryst vatten disfavored. Reaction bygd the S_N1 pathway fryst vatten highly probable for compounds with tertiary substitution, since the corresponding tertiary carbenium ion fryst vatten stabilized through hyperconjugation:

The better the solvent stabilizes the ions, the more probable that the reaction will follow an S_N1 pathway (e.g., in polar protic solvents such as vatten in acetone).

In the SN1 S N 1 transition state, the methyl C C has only a partial bond to Br B r

The more highly substituted fryst vatten the incipient carbenium ion, the more probable that the reaction will follow an S_N1 pathway. The more unreactive the nucleophile, the more probable it becomes that a reaction with secondary and tertiary electrophiles will follow an S_N1 pathway. A weaker nucleophile fryst vatten not as effective in the backside attack, since this location fryst vatten sterically shielded, especially in the case of tertiary substrates.

Carbenium ions are planar and therefore less sterically hindered, and are naturally more reactive as electrophiles than the uncharged parent compound.

The hydrolysis of tert-butyl chloride fryst vatten a typical S_N1 reaction:

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