Why sn2 aprotic

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Mechanism of substitution at a saturated carbon atom. Part XXVI. Section A introductory remarks, and a kinetic study of the reactions of methyl, ethyl, n-propyl, isobutyl, and neopentyl bromides with sodium ethoxide in dry ethyl alcohol I.

Dostrovsky and E. Hughes J. This can be attributed to sterics, as backside attack of the substituted carbon becomes increasingly challenging. Part III. Kinetics of the degradations of sulphonium compounds John L. Gleave, Edward D. Hughes and Christopher K.

Ingold J. Influence of poles and polar linkings on the course pursued by elimination reactions. Part XVI. Mechanism of the thermal decomposition of quaternary ammonium compounds E. Ingold, and C. Patel J. Basically, the S N 1 and S N 2 mechanisms as taught are two extremes of a continuum, and in practice most reactions lie somewhere in between. Part IX. Bateman and Edward D. Whitmore, E. Wittle, and A. In this case, the neopentyl cation quickly rearranges to the significantly more stable t -amyl cation, and those products are obtained.

Reaction kinetics and the Walden inversion. Part I. Hughes, Christopher K. Ingold and Standish Masterman J. Part IV. Are Acids! What Holds The Nucleus Together? Each done on a powerpoint slide with a pretty background… Leaving group breaks off Forming carbocation SN1, first step very reactive intermediate species they need electrons tertiary good hyperconjugation helps resonance does too add more Nu?

No help. SN1 or SN2 there are many factors. This was so helpful!! I love how simple you break it down. Awesome explaination with some simple but effective intellectual ideas!!!!!! Thanks so much! Hey in SN1 why tertiary is more reactive though it is relatively stable.

Here what do you mean by inversion of configuration. Is it relative or absolute. Which one allows for a better control over the configurations of products SN1 or SN2? Great analogy and summary! Thank you so much! I shall definitely be reading more of your work! Great explanation.. Yes, it can certainly happen through a simultaneous hydride shift. Small nucleophiles react more rapidly than sterically-demanding nucleophiles.

Stable anions are good leaving groups. If the leaving group is a stable anion, the transition state's energy is lower than with a leaving group that is a less stable anion. Consequently, the activation energy is lower, and the reaction rate is higher.

The larger the area in which the negative charge is located is, or the more delocalized the negative charge is, the more stable anions are. Therefore, iodide anions, for instance, are better leaving groups than chloride anions are. On a laboratory-scale and large-scale, tosylates and mesylates are often applied to organic syntheses. In the transition state, their partial negative charge is particularly well delocalized, so that the transition state's energy is considerably low.

Fluoride anions, hydroxide anions, alkoxides, and amide anions are poor leaving groups. Therefore, S N 2 reactions with fluoroalkanes, alcohols, ethers, or amines virtually never occur. However, branching at carbons farther away from the electrophilic carbon would have a much smaller effect.

Nucleophilic functional groups are those which have electron-rich atoms able to donate a pair of electrons to form a new covalent bond. In both laboratory and biological organic chemistry, the most relevant nucleophilic atoms are oxygen, nitrogen, and sulfur, and the most common nucleophilic functional groups are water, alcohols, phenols, amines, thiols, and occasionally carboxylates.

More specifically in laboratory reactions, halide and azide N 3 - anions are commonly seen acting as nucleophiles. When thinking about nucleophiles, the first thing to recognize is that, for the most part, the same quality of 'electron-richness' that makes a something nucleophilic also makes it basic: nucleophiles can be bases, and bases can be nucleophiles. It should not be surprising, then, that most of the trends in basicity that we have already discussed also apply to nucleophilicity.

Some confusion in distinguishing basicity base strength and nucleophilicity nucleophile strength is inevitable. Since basicity is a less troublesome concept; it is convenient to start with it.

Basicity refers to the ability of a base to accept a proton. Basicity may be related to the pKa of the corresponding conjugate acid, as shown below.

The strongest bases have the weakest conjugate acids and vice versa. The range of basicities included in the following table is remarkable, covering over fifty powers of ten! In an acid-base equilibrium the weakest acid and the weakest base will predominate they will necessarily be on the same side of the equilibrium. Learning the pKa values for common compounds provides a useful foundation on which to build an understanding of acid-base factors in reaction mechanisms.

Nucleophilicity is a more complex property. It commonly refers to the rate of substitution reactions at the halogen-bearing carbon atom of a reference alkyl halide, such as CH 3 -Br.

Thus the nucleophilicity of the Nu: — reactant in the following substitution reaction varies as shown in the chart below:. There are predictable periodic trends in nucleophilicity. Moving horizontally across the second row of the table, the trend in nucleophilicity parallels the trend in basicity:.

The reasoning behind the horizontal nucleophilicity trend is the same as the reasoning behind the basicity trend: more electronegative elements hold their electrons more tightly, and are less able to donate them to form a new bond. This horizontal trend also tells us that amines are more nucleophilic than alcohols, although both groups commonly act as nucleophiles in both laboratory and biochemical reactions.

Recall that the basicity of atoms decreases as we move vertically down a column on the periodic table: thiolate ions are less basic than alkoxide ions, for example, and bromide ion is less basic than chloride ion, which in turn is less basic than fluoride ion.

Recall also that this trend can be explained by considering the increasing size of the 'electron cloud' around the larger ions: the electron density inherent in the negative charge is spread around a larger area, which tends to increase stability and thus reduce basicity. The vertical periodic trend for nucleophilicity is somewhat more complicated that that for basicity: depending on the solvent that the reaction is taking place in, the nucleophilicity trend can go in either direction.

Let's take the simple example of the SN2 reaction below:. If this reaction is occurring in a protic solvent that is, a solvent that has a hydrogen bonded to an oxygen or nitrogen - water, methanol and ethanol are the most important examples , then the reaction will go fastest when iodide is the nucleophile, and slowest when fluoride is the nucleophile, reflecting the relative strength of the nucleophile.

This of course, is opposite that of the vertical periodic trend for basicity, where iodide is the least basic. What is going on here?

Shouldn't the stronger base, with its more reactive unbonded valence electrons, also be the stronger nucleophile? As mentioned above, it all has to do with the solvent. Remember, we are talking now about the reaction running in a protic solvent like ethanol.

Protic solvent molecules form very strong ion-dipole interactions with the negatively-charged nucleophile, essentially creating a 'solvent cage' around the nucleophile:. In order for the nucleophile to attack the electrophile, it must break free, at least in part, from its solvent cage.

The lone pair electrons on the larger, less basic iodide ion interact less tightly with the protons on the protic solvent molecules - thus the iodide nucleophile is better able to break free from its solvent cage compared the smaller, more basic fluoride ion, whose lone pair electrons are bound more tightly to the protons of the cage.

The picture changes if we switch to a polar aprotic solvent , such as acetone, in which there is a molecular dipole but no hydrogens bound to oxygen or nitrogen. Now, fluoride is the best nucleophile, and iodide the weakest.