Arrow pushing
Encyclopedia
Arrow pushing or electron pushing is a technique used to describe the progression of organic chemistry
reaction
mechanisms. In using arrow pushing, "curved arrows" or "curly arrows" are superimposed over the structural formulae of reactants in a chemical equation
to show the reaction mechanism
. The arrows illustrate the movement of electron
s as bonds
between atom
s are broken and formed. Arrow pushing is also used to describe how positive and negative charges
are distributed around organic molecules through resonance
.
When a bond is broken, electrons leave where the bond was and this is represented by a curved arrow pointing away from the bond and ending the arrow pointing towards the next unoccupied molecular orbital . Similarly, organic chemists represent the formation of a bond by a curved arrow pointing between two species.
For clarity, when pushing arrows, it is best to draw the arrows starting from a lone pair of electrons or filled bonds (sigma, pi) and ending in an unfilled molecular orbital, allowing the reader to know exactly which electrons are moving and where they are ending.
Homolytic bond cleavage
is a process where the electron pair comprising a bond is split, causing the bond to break. This is denoted by two single barbed curved arrows pointing away from the bond. The consequence of this process is the retention of a single unpaired electron on each of the atoms that were formerly joined by a bond. These single electron species are known as free radicals.
(an anion) and a positively charged species (a cation). The anion is the species that retains the electrons from the bond while the cation is stripped of the electrons from the bond.
occurs when a molecule separates into a positively charged component and a negatively charged component. This generally occurs in highly polar solvent
s through a process called solvolysis
. The positively charged component then reacts with a nucleophile
forming a new compound.
In the first stage of this reaction (solvolysis), the C-L bond breaks and both electrons from that bond join L (the leaving group
) to form L- and R3C+ ions. This is represented by the curved arrow pointing away from the C-L bond and towards L. The nucleophile Nu-, being attracted to the R3C+, then donates a pair of electrons forming a new C-Nu bond.
Because an SN1 reaction proceeds with the Substitution of a leaving group with a Nucleophile, the SN designation is used. Because the initial solvolysis step in this reaction involves a single molecule dissociating from its leaving group, the initial stage of this process is considered a uni-molecular reaction. The involvement of only 1 species in the initial phase of the reaction enhances the mechanistic designation to SN1.
occurs when a nucleophile displaces a leaving group residing on a molecule. This displacement or substitution results in the formation of a new compound.
Because an SN2 reaction proceeds with the Substitution of a leaving group with a Nucleophile, the SN designation is used. Because this mechanism proceeds with the initial approach of two species, it is referred to as a bi-molecular reaction. The involvement of 2 species enhances the mechanistic designation to SN2.
occurs when a proton
adjacent to a positive charge leaves and generates a double bond
.
Because initial formation of a cation is necessary for E1 reactions to occur, E1 reactions are often observed as side reactions to SN1 mechanisms.
E1 eliminations proceed with the Elimination of a leaving group leading to the E designation. Because this mechanism proceeds with the initial dissociation of a single starting material forming a carbocation, this process is considered a uni-molecular reaction. The involvement of only 1 species in the initial phase of the reaction enhances the mechanistic designation to E1.
with simultaneous elimination of a leaving group and generation of a double bond.
Similar to the relationship between E1 eliminations and SN1 mechanisms, E2 eliminations often occur in competition with SN2 reactions. This observation is most often noted when the base
is also a nucleophile. In order to minimize this competition, non-nucleophilic bases are commonly used to effect E2 eliminations.
E2 eliminations proceed through initial extraction of a proton by a base or nucleophile leading to Elimination of a leaving group justifying the E designation. Because this mechanism proceeds through the interaction of two species (substrate and base/nucleophile), E2 reactions are recognized as bi-molecular. Thus, the involvement of 2 species in the initial phase of the reaction enhances the mechanistic designation to E2.
s occur when nucleophiles react with carbonyl
s. When a nucleophile adds to a simple aldehyde
or ketone
, the result is a 1,2-addition. When a nucleophile adds to a conjugated carbonyl system, the result is a 1,4-addition. The designations 1,2 and 1,4 are derived from numbering the atoms of the starting compound where the oxygen is labeled “1” and each atom adjacent to the oxygen are sequentially numbered out to the site of nucleophilic addition. A 1,2-addition occurs with nucleophilic addition to position 2 while a 1,4-addition occurs with nucleophilic addition to position 4.
s (or related functional groups) react with nucleophiles. In fact, the only requirement for an addition-elimination reaction to proceed is that the group being eliminated is a better leaving group than the incoming nucleophile.
Organic chemistry
Organic chemistry is a subdiscipline within chemistry involving the scientific study of the structure, properties, composition, reactions, and preparation of carbon-based compounds, hydrocarbons, and their derivatives...
reaction
Chemical reaction
A chemical reaction is a process that leads to the transformation of one set of chemical substances to another. Chemical reactions can be either spontaneous, requiring no input of energy, or non-spontaneous, typically following the input of some type of energy, such as heat, light or electricity...
mechanisms. In using arrow pushing, "curved arrows" or "curly arrows" are superimposed over the structural formulae of reactants in a chemical equation
Chemical equation
A chemical equation is the symbolic representation of a chemical reaction where the reactant entities are given on the left hand side and the product entities on the right hand side. The coefficients next to the symbols and formulae of entities are the absolute values of the stoichiometric numbers...
to show the reaction mechanism
Reaction mechanism
In chemistry, a reaction mechanism is the step by step sequence of elementary reactions by which overall chemical change occurs.Although only the net chemical change is directly observable for most chemical reactions, experiments can often be designed that suggest the possible sequence of steps in...
. The arrows illustrate the movement of electron
Electron
The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...
s as bonds
Chemical bond
A chemical bond is an attraction between atoms that allows the formation of chemical substances that contain two or more atoms. The bond is caused by the electromagnetic force attraction between opposite charges, either between electrons and nuclei, or as the result of a dipole attraction...
between atom
Atom
The atom is a basic unit of matter that consists of a dense central nucleus surrounded by a cloud of negatively charged electrons. The atomic nucleus contains a mix of positively charged protons and electrically neutral neutrons...
s are broken and formed. Arrow pushing is also used to describe how positive and negative charges
Electric charge
Electric charge is a physical property of matter that causes it to experience a force when near other electrically charged matter. Electric charge comes in two types, called positive and negative. Two positively charged substances, or objects, experience a mutual repulsive force, as do two...
are distributed around organic molecules through resonance
Resonance (chemistry)
In chemistry, resonance or mesomerism is a way of describing delocalized electrons within certain molecules or polyatomic ions where the bonding cannot be expressed by one single Lewis formula...
.
Notation
Organic chemists use two types of arrows within molecular structures to describe electron movements. Single electrons trajectories are designated with single barbed arrows, whereas double-barbed arrows show movement of electron pairs.When a bond is broken, electrons leave where the bond was and this is represented by a curved arrow pointing away from the bond and ending the arrow pointing towards the next unoccupied molecular orbital . Similarly, organic chemists represent the formation of a bond by a curved arrow pointing between two species.
For clarity, when pushing arrows, it is best to draw the arrows starting from a lone pair of electrons or filled bonds (sigma, pi) and ending in an unfilled molecular orbital, allowing the reader to know exactly which electrons are moving and where they are ending.
Breaking of bonds
A covalent bond joining atoms in an organic molecule consists of a group of two electrons. Such a group is referred to as an electron pair. Reactions in organic chemistry proceed through the sequential breaking and formation of such bonds. Organic chemists recognize two processes for the breaking of a chemical bond. These processes are known as homolytic cleavage and heterolytic cleavage.Homolytic bond cleavage
Bond cleavage
Bond cleavage, or scission, is the splitting of chemical bonds.If the two electrons in a cleaved covalent bond are divided between the products, the process is known as homolytic fission and free redicals are generated by homolytic cleavage the process is known as homolytic fission or homolysis...
is a process where the electron pair comprising a bond is split, causing the bond to break. This is denoted by two single barbed curved arrows pointing away from the bond. The consequence of this process is the retention of a single unpaired electron on each of the atoms that were formerly joined by a bond. These single electron species are known as free radicals.
Heterolytic bond cleavage
Heterolytic bond cleavage is a process where the electron pair that comprised a bond moves to one of the atoms that was formerly joined by a bond. The bond breaks, forming a negatively charged speciesChemical species
Chemical species are atoms, molecules, molecular fragments, ions, etc., being subjected to a chemical process or to a measurement. Generally, a chemical species can be defined as an ensemble of chemically identical molecular entities that can explore the same set of molecular energy levels on a...
(an anion) and a positively charged species (a cation). The anion is the species that retains the electrons from the bond while the cation is stripped of the electrons from the bond.
Heterolytic Reaction Mechanisms
All heterolytic organic chemistry reactions can be described by a sequence of fundamental mechanistic subtypes. The elementary mechanistic subtypes taught in introductory organic chemistry are SN1, SN2, E1, E2, addition and addition-elimination. Using arrow pushing, each of these mechanistic subtypes can be described.SN1 reactions
An SN1 reactionSN1 reaction
The SN1 reaction is a substitution reaction in organic chemistry. "SN" stands for nucleophilic substitution and the "1" represents the fact that the rate-determining step is unimolecular...
occurs when a molecule separates into a positively charged component and a negatively charged component. This generally occurs in highly polar solvent
Solvent
A solvent is a liquid, solid, or gas that dissolves another solid, liquid, or gaseous solute, resulting in a solution that is soluble in a certain volume of solvent at a specified temperature...
s through a process called solvolysis
Solvolysis
Solvolysis is a special type of nucleophilic substitution or elimination where the nucleophile is a solvent molecule. For certain nucleophiles, there are specific terms for the type of solvolysis reaction...
. The positively charged component then reacts with a nucleophile
Nucleophile
A nucleophile is a species that donates an electron-pair to an electrophile to form a chemical bond in a reaction. All molecules or ions with a free pair of electrons can act as nucleophiles. Because nucleophiles donate electrons, they are by definition Lewis bases.Nucleophilic describes the...
forming a new compound.
In the first stage of this reaction (solvolysis), the C-L bond breaks and both electrons from that bond join L (the leaving group
Leaving group
In chemistry, a leaving group is a molecular fragment that departs with a pair of electrons in heterolytic bond cleavage. Leaving groups can be anions or neutral molecules. Common anionic leaving groups are halides such as Cl−, Br−, and I−, and sulfonate esters, such as para-toluenesulfonate...
) to form L- and R3C+ ions. This is represented by the curved arrow pointing away from the C-L bond and towards L. The nucleophile Nu-, being attracted to the R3C+, then donates a pair of electrons forming a new C-Nu bond.
Because an SN1 reaction proceeds with the Substitution of a leaving group with a Nucleophile, the SN designation is used. Because the initial solvolysis step in this reaction involves a single molecule dissociating from its leaving group, the initial stage of this process is considered a uni-molecular reaction. The involvement of only 1 species in the initial phase of the reaction enhances the mechanistic designation to SN1.
SN2 reactions
An SN2 reactionSN2 reaction
The SN2 reaction is a type of nucleophilic substitution, where a lone pair from a nucleophile attacks an electron deficient electrophilic center and bonds to it, expelling another group called a leaving group. Thus the incoming group replaces the leaving group in one step...
occurs when a nucleophile displaces a leaving group residing on a molecule. This displacement or substitution results in the formation of a new compound.
Because an SN2 reaction proceeds with the Substitution of a leaving group with a Nucleophile, the SN designation is used. Because this mechanism proceeds with the initial approach of two species, it is referred to as a bi-molecular reaction. The involvement of 2 species enhances the mechanistic designation to SN2.
E1 eliminations
An E1 eliminationElimination reaction
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...
occurs when a proton
Proton
The proton is a subatomic particle with the symbol or and a positive electric charge of 1 elementary charge. One or more protons are present in the nucleus of each atom, along with neutrons. The number of protons in each atom is its atomic number....
adjacent to a positive charge leaves and generates a double bond
Double bond
A double bond in chemistry is a chemical bond between two chemical elements involving four bonding electrons instead of the usual two. The most common double bond, that between two carbon atoms, can be found in alkenes. Many types of double bonds between two different elements exist, for example in...
.
Because initial formation of a cation is necessary for E1 reactions to occur, E1 reactions are often observed as side reactions to SN1 mechanisms.
E1 eliminations proceed with the Elimination of a leaving group leading to the E designation. Because this mechanism proceeds with the initial dissociation of a single starting material forming a carbocation, this process is considered a uni-molecular reaction. The involvement of only 1 species in the initial phase of the reaction enhances the mechanistic designation to E1.
E2 eliminations
An E2 elimination occurs when a proton adjacent to a leaving group is extracted by a baseBase (chemistry)
For the term in genetics, see base A base in chemistry is a substance that can accept hydrogen ions or more generally, donate electron pairs. A soluble base is referred to as an alkali if it contains and releases hydroxide ions quantitatively...
with simultaneous elimination of a leaving group and generation of a double bond.
Similar to the relationship between E1 eliminations and SN1 mechanisms, E2 eliminations often occur in competition with SN2 reactions. This observation is most often noted when the base
Base (chemistry)
For the term in genetics, see base A base in chemistry is a substance that can accept hydrogen ions or more generally, donate electron pairs. A soluble base is referred to as an alkali if it contains and releases hydroxide ions quantitatively...
is also a nucleophile. In order to minimize this competition, non-nucleophilic bases are commonly used to effect E2 eliminations.
E2 eliminations proceed through initial extraction of a proton by a base or nucleophile leading to Elimination of a leaving group justifying the E designation. Because this mechanism proceeds through the interaction of two species (substrate and base/nucleophile), E2 reactions are recognized as bi-molecular. Thus, the involvement of 2 species in the initial phase of the reaction enhances the mechanistic designation to E2.
Addition reactions
Addition reactionAddition reaction
An addition reaction, in organic chemistry, is in its simplest terms an organic reaction where two or more molecules combine to form a larger one....
s occur when nucleophiles react with carbonyl
Carbonyl
In organic chemistry, a carbonyl group is a functional group composed of a carbon atom double-bonded to an oxygen atom: C=O. It is common to several classes of organic compounds, as part of many larger functional groups....
s. When a nucleophile adds to a simple aldehyde
Aldehyde
An aldehyde is an organic compound containing a formyl group. This functional group, with the structure R-CHO, consists of a carbonyl center bonded to hydrogen and an R group....
or ketone
Ketone
In organic chemistry, a ketone is an organic compound with the structure RCR', where R and R' can be a variety of atoms and groups of atoms. It features a carbonyl group bonded to two other carbon atoms. Many ketones are known and many are of great importance in industry and in biology...
, the result is a 1,2-addition. When a nucleophile adds to a conjugated carbonyl system, the result is a 1,4-addition. The designations 1,2 and 1,4 are derived from numbering the atoms of the starting compound where the oxygen is labeled “1” and each atom adjacent to the oxygen are sequentially numbered out to the site of nucleophilic addition. A 1,2-addition occurs with nucleophilic addition to position 2 while a 1,4-addition occurs with nucleophilic addition to position 4.
Addition-elimination reactions
Addition-elimination reactions are addition reactions immediately followed by elimination reactions. In general, these reactions take place when esterEster
Esters are chemical compounds derived by reacting an oxoacid with a hydroxyl compound such as an alcohol or phenol. Esters are usually derived from an inorganic acid or organic acid in which at least one -OH group is replaced by an -O-alkyl group, and most commonly from carboxylic acids and...
s (or related functional groups) react with nucleophiles. In fact, the only requirement for an addition-elimination reaction to proceed is that the group being eliminated is a better leaving group than the incoming nucleophile.
See also
- Organic chemistryOrganic chemistryOrganic chemistry is a subdiscipline within chemistry involving the scientific study of the structure, properties, composition, reactions, and preparation of carbon-based compounds, hydrocarbons, and their derivatives...
- Organic reactionOrganic reactionOrganic reactions are chemical reactions involving organic compounds. The basic organic chemistry reaction types are addition reactions, elimination reactions, substitution reactions, pericyclic reactions, rearrangement reactions, photochemical reactions and redox reactions. In organic synthesis,...
- Reaction mechanismReaction mechanismIn chemistry, a reaction mechanism is the step by step sequence of elementary reactions by which overall chemical change occurs.Although only the net chemical change is directly observable for most chemical reactions, experiments can often be designed that suggest the possible sequence of steps in...
- SN1 reactionSN1 reactionThe SN1 reaction is a substitution reaction in organic chemistry. "SN" stands for nucleophilic substitution and the "1" represents the fact that the rate-determining step is unimolecular...
- SN2 reactionSN2 reactionThe SN2 reaction is a type of nucleophilic substitution, where a lone pair from a nucleophile attacks an electron deficient electrophilic center and bonds to it, expelling another group called a leaving group. Thus the incoming group replaces the leaving group in one step...
- Elimination reactionElimination reactionAn 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...
External links
- Study Organic Chemistry, Resources for Success in Organic Chemistry
- MIT.edu, OpenCourseWare: Organic Chemistry I
- HaverFord.edu, Organic Chemistry Lectures, Videos and Text
- Organic-Chemistry.org, Organic Chemistry Portal - Recent Abstracts and (Name)Reactions
- Ochem4free.info, Home of a full, online, peer-reviewed organic chemistry text
- CEM.MSU.edu, Virtual Textbook of Organic Chemistry
- Organic Chemistry Resources WorldWide - A collection of Links
- Thinkquest.org, Organic Families and Their Functional Groups
- OrgChemInfo.8k.com, A collection of Organic chemistry Resources