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Encyclopedia > SN1 reaction

The SN1 reaction is an substitution reaction in organic chemistry. "SN" stands for substitution nucleophilic and the "1" represents the fact that the rate-determining step is unimolecular. It involves a carbocation intermediate, and it is commonly seen in reactions of secondary or tertiary alkyl halides, or (under strongly acidic conditions) with secondary or tertiary alcohols. With primary alkyl halides, the alternative mechanism of the SN2 reaction occurs instead. Among inorganic chemists, SN1 is referred to perhaps more accessibly as a dissociative mechanism. In a substitution reaction, a group of a particular compound is replaced by another group. ... Organic chemistry is the scientific study of the structure, properties, composition, reactions, and synthesis of organic compounds that by definition contain carbon. ... In chemistry, nucleophilic substitution is a class of substitution reaction in which an electron-rich nucleophile attacks a molecule and replaces a group or atom, called the leaving group. ... The rate-determining step is a chemistry term for the slowest step in a chemical reaction. ... In general, a molecule is the smallest particle of a pure chemical substance that still retains its composition and chemical properties. ... A carbocation is an ion with a positively-charged carbon atom. ... In chemistry, an alkyl halide is an organic molecule of the form R_X, where X is a halide and R contains a carbon atom bonded to other functional groups or hydrogens. ... In chemistry, alcohol is any organic compound in which a hydroxyl group (-OH) is bound to a carbon atom, which in turn is bound to other hydrogen and/or carbon atoms. ... The SN2 reaction is a type of nucleophilic substitution, where a nucleophile attacks an electrophilic center and bonds to it, expelling another group called a leaving group. ...

Contents


Mechanism

The SN1 reaction between a molecule A and a nucleophile B takes place in two steps: Image File history File links Download high resolution version (950x760, 119 KB) Summary Diagram of the SN1 mechanistic steps for hydrolysis of an alkyl halide. ...

  1. Formation of a carbocation from molecule A by separation of leaving group from the carbon.
  2. Nucleophilic attack: a nucleophile (molecule B) joins onto the carbon from A. If the nucleophile B is a neutral molecule (very often it is a solvent molecule such as water), a third step is required to complete the reaction.
  3. Deprotonation: Removal of a proton on the protonated nucleophile by a nearby ion or molecule.

An example reaction: A carbocation is an ion with a positively-charged carbon atom. ... A leaving group is an atom or molecule that detaches from an organic molecule, which, after detachment, is called the residual or main part. ... In chemistry, a nucleophile (literally nucleus lover) is a reagent which is attracted to centres of positive charge. ... Deprotonation is a chemistry term that refers to the removal of a proton (hydrogen ion H+) from a molecule, forming the conjugate base. ...

(CH3)3CBr + H2O → (CH3)3COH + HBr

This goes via the three step reaction mechanism described above: Water has the chemical formula H2O, meaning that one molecule of water is composed of two hydrogen atoms and one oxygen atom. ... Butanol is a higher alcohol with a 4 carbon atoms and a general formula of C4H10O. There are 4 different isomeric structures for butanol: butan-1-ol CH3-CH2-CH2-CH2-OH butan-2-ol CH3-CH2-CH(OH)-CH3 sec-butanol CH3-CH-CH3 | CH2OH OH | tert-butanol CH3... Hydrobromic Acid is a chemical compound, with the chemical formula HBr. ... In chemistry, a reaction mechanism is the step by step sequence of elementary reactions by which overall chemical change occurs. ...

  1. (CH3)3CBr → (CH3)3C+ + Br
  2. (CH3)3C+ + H2O → (CH3)3C-OH2+
  3. (CH3)3C-OH2+ + H2O → (CH3)3COH + H3O+

General Name, Symbol, Number bromine, Br, 35 Chemical series halogens Group, Period, Block 17, 4, p Appearance gas/liquid: red-brown solid: metallic luster Atomic mass 79. ...

Kinetics

In contrast to SN2, SN1 reactions take place in two steps (excluding any protonation or deprotonation). The rate determining step is the first step, so the rate of the overall reaction is essentially equal to that of carbocation formation, and does not involve the attacking nucleophile. Thus nucleophilicity is irrelevant and the overall reaction rate depends on the concentration of the substrate only. Kinetics refers to two different areas of science: Chemical kinetics studies reaction rates. ... The SN2 reaction is a type of nucleophilic substitution, where a nucleophile attacks an electrophilic center and bonds to it, expelling another group called a leaving group. ... Protonation is the addition of a proton (H+) to an atom, molecule, or ion. ... Deprotonation is a chemistry term that refers to the removal of a proton (hydrogen ion H+) from a molecule, forming the conjugate base. ... The rate-determining step is a chemistry term for the slowest step in a chemical reaction. ... A carbocation is an ion with a positively-charged carbon atom. ... In chemistry, a nucleophile (literally nucleus lover) is a reagent which is attracted to centres of positive charge. ... The reaction rate for a reactant or product in a particular reaction is defined as the amount (in moles or mass units) per unit time per unit volume that is formed or removed. ... The word substrate can mean the following: In biochemistry, a substrate is a molecule which is acted upon by an enzyme. ...

rate = k[RX]

In some cases the SN1 reaction will occur at an abnormally high rate due to Neighbouring group participation, NGP often lowers the energy barrier required for the formation of the carbocation intermidate. Neighbouring group participation or NGP in organic chemistry has been defined by IUPAC as the interaction of a reaction centre with a lone pair of electrons in an atom or the electrons present in a sigma bond or pi bond . ... A carbocation is an ion with a positively-charged carbon atom. ...


Scope of the reaction

The SN1 mechanism tends to predominate when the central carbon atom is surrounded by bulky groups because such groups interfere sterically with the SN2 reaction. In addition bulky substituents on the central carbon increase the rate of carbocation formation because of the relief of steric strain that occurs. The resultant carbocation is also stabilized by both inductive stabilisation and hyperconjugation from attached alkyl groups, and the Hammond-Leffler postulate suggests that this too will increase the rate of carbocation formation. The SN1 mechanism therefore dominates in reactions at tertiary alkyl centres, and it is also observed at secondary alkyl centres when weak nucleophiles are used. General Name, Symbol, Number carbon, C, 6 Chemical series nonmetals Group, Period, Block 14, 2, p Appearance black (graphite) colorless (diamond) Atomic mass 12. ... Properties In chemistry and physics, an atom (Greek άτομον meaning indivisible) is the smallest possible particle of a chemical element that retains its chemical properties. ... Steric effects are the interaction of molecules dictated by their shape and/or spatial relationships. ... The reaction rate for a reactant or product in a particular reaction is defined as the amount (in moles or mass units) per unit time per unit volume that is formed or removed. ... A carbocation is an ion with a positively-charged carbon atom. ... In chemistry van der Waals strain is strain resulting from van der Waals repulsion when two substituents in a molecule approach each other with a distance less than the sum of their van der Waals radii. ... The inductive effect is associated with the dipole moment of the compound R-X. If X is at the negative end of a dipole moment, it will draw electrons from the ring and produce -I effect. ... ( Hyperconjugation is the stabilising interaction that results from the interaction of the electrons in a s-bond (usually C-H or C-C) with an adjacent empty (or partially filled) p-orbital or a p-orbital to give an extended molecular orbital that increases the stability of the system. ... An Alkyl is a univalent radical containing only carbon and hydrogen atoms arranged in a chain. ... The Hammond-Leffler Postulate was formulated by G. S. Hammond (then at the California Institute of Technology) and J. E. Leffler (Florida State University) and states that the structure of a transition state resembles that of the species nearest to it in free energy. ... In chemistry, a nucleophile (literally nucleus lover) is a reagent which is attracted to centres of positive charge. ...


Stereochemistry

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 the event of the molecule chirality. In fact, if the central carbon is the only stereocenter in the reaction, racemization may occur. However, an excess of inversion is usually observed, as the leaving group can remain in close proximity to the carbocation intermediate for a short time and block nucleophilic attack. For example, in the reaction of 3S-chloro-3-methylhexane with iodide ion, if the carbocation intermediate is free of the leaving group then it is achiral and stands an equal chance of attack on either side. This leads to a mixture of 3R-iodo-3-methylhexane and 3S-iodo-3-methylhexane: The different types of isomers. ... A stereocenter in organic chemistry generally refers to a carbon atom in a chemical compound that has four different types of atoms or groups of atoms attached to it. ... In chemistry, a nucleophile (literally nucleus lover) is a reagent which is attracted to centres of positive charge. ... Main article: stereochemistry Stereoisomerism is the arrangement of atoms in molecules whose connectivity remains the same but their arrangement in space is different in each isomer. ... In chemistry racemization refers to partial conversion of one enantiomer into another. ... An iodide ion is an iodine atom with a -1 (negative one) charge. ...

Image File history File links SN1Stereochemistry. ...

Side reactions

Two common side reactions are elimination reactions and carbocation rearrangement. If the reaction is performed under warm or hot conditions (which favor an increase in entropy), E1 elimination is likely to predominate, leading to formation of an alkene. Even if the reaction is performed cold, some alkene may be formed. If an attempt is made to perform an SN1 reaction using a strongly basic nucleophile such as hydroxide or methoxide ion, the alkene will again be formed, this time via an E2 elimination. This will be especially true if the reaction is heated. Finally, if the carbocation intermediate can rearrange to a more stable carbocation, it will give a product derived from the more stable carbocation rather than the simple substitution product. An elimination reaction is a type of organic reaction in which two substituents are removed from a molecule in either a one or two-step mechanism. ... An elimination reaction is a type of organic reaction in which two substituents are removed from a molecule in either a one or two-step mechanism. ... An alkene in organic chemistry is an unsaturated hydrocarbon containing at least one carbon to carbon double bond. ... Hydroxide is a polyatomic ion consisting of oxygen and hydrogen: OH− It has a charge of −1. ... Methoxide is an organic salt, in pure form a white powder. ... An elimination reaction is a type of organic reaction in which two substituents are removed from a molecule in either a one or two-step mechanism. ...


Solvent effects

Since the SN1 reaction involves formation of an unstable carbocation intermediate in the rate-determining step, anything that can facilitate this will speed up the reaction. The normal solvents of choice are both polar (to stabilise ionic intermediates in general) and protic (to solvate the leaving group in particular). Typical polar protic solvents include water and alcohols, which will also act as nucleophiles. layers]] that separate organs or subcompartments in organisms. ... In chemistry any solvent that carries hydrogen attached to oxygen as in a hydroxyl group, nitrogen as in a amine group, or, more generally, any molecular solvent which contains dissociable H+, such as hydrogen fluoride, is called a protic solvent. ... Solvation is the attraction and association of molecules of a solvent with molecules or ions of a solute. ...


See also

In a substitution reaction, a group of a particular compound is replaced by another group. ... In chemistry, nucleophilic substitution is a class of substitution reaction in which an electron-rich nucleophile attacks a molecule and replaces a group or atom, called the leaving group. ... The SN2 reaction is a type of nucleophilic substitution, where a nucleophile attacks an electrophilic center and bonds to it, expelling another group called a leaving group. ... SNi or Substitution Nucleophilic internal stands for a specific but not often encountered nucleophilic aliphatic substitution reaction mechanism. ... Nucleophilic acyl substitution is a type of substitution reaction between nucleophiles and acyl compounds. ... Neighbouring group participation or NGP in organic chemistry has been defined by IUPAC as the interaction of a reaction centre with a lone pair of electrons in an atom or the electrons present in a sigma bond or pi bond . ...

External links

  • Diagrams: Frostburg State University
  • Exercise: the University of Maine

References

  1. L. G. Wade, Jr., Organic Chemistry, 6th ed., Pearson/Prentice Hall, Upper Saddle River, New Jersey, USA, 2005.
  2. J. March, Advanced Organic Chemistry, 4th ed., Wiley, New York, 1992.

  Results from FactBites:
 
SN1 reaction - Wikipedia, the free encyclopedia (772 words)
1 reaction is an substitution reaction in organic chemistry.
It involves a carbocation intermediate, and it is commonly seen in reactions of secondary or tertiary alkyl halides, or (under strongly acidic conditions) with secondary or tertiary alcohols.
If the reaction is performed under warm or hot conditions (which favor an increase in entropy), E1 elimination is likely to predominate, leading to formation of an alkene.
Nucleophilic substitution - Wikipedia, the free encyclopedia (725 words)
In chemistry, nucleophilic substitution is a class of substitution reaction in which an electron-rich nucleophile attacks a molecule and replaces a group or atom, called the leaving group.
It is a fundamental class of reaction in organic chemistry, where the reaction occurs at a carbon centre, but nucleophilic substitutions are also well known in inorganic covalent compounds too.
R-Br + OH → R-OH + Br Nucleophilic substitution reactions are commonplace in organic chemistry, and they can be broadly categorised as taking place at an aliphatic (saturated) carbon or at (less often) an aromatic or other unsaturated carbon centre.
  More results at FactBites »

 
 

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