stayOrganic compoundIn molecules, due to the influence of substituents (atoms or atomic clusters) with different electronegativity, the density of bonding electron cloud in the whole molecule shifts in a certain direction, which makes the molecule polarized, called induction effect.The inductive effect shown by the polar bond is called static inductive effect, andChemical reaction processThe induced effect of polarization bond caused by external electric field (such as reagent and solvent) is called dynamic induced effect.The inductive effect only changes the density distribution of the electron cloud in the bond, but does not change the nature of the bond.Compared with the conjugate effect, there is no polarity alternation.
Chinese name
Induced effect
Foreign name
inductive effect
Applied discipline
Organic chemistry
Essence
Changing the electron cloud density distribution in the bond
The induced effect is characterized by the electron cloud migration transferring along the σ bond, weakening with the growth of the carbon chain, and finally disappearing.The induction effect is a short-range force, which is very weak when it is transferred to the third carbon, and almost disappears when it is transferred to the fifth carbon.Induction effect is a kind of electrostatic effect, which is permanent and belongs to electronic effect.[1]
Representation method
In the induction effect, the arrow "→" is generally used to indicate the direction of electron movementElectronic cloudThe distribution of has changed.For example, after the chlorine atom replaces the hydrogen atom on the alkane carbon, as shown in Figure 1:
Figure 1
Due to the large electronegativity of chlorine, it has a strong ability to attract electrons. The electrons shift to chlorine, making chlorine partially negatively charged (δ-）Positive charge of carbon belt (δ+）。The partially positively charged carbon attracts the electrons on the adjacent carbon to offset it.
type
Static induced effect: due to intramolecularPolar covalent bondThe inherent properties of static molecules caused by the existence of (internal electric field).The influence on the reaction activity of compounds has two sides. Under certain conditions, the reaction activity can be increased or decreased.
Dynamic induction effect: it occurs during chemical reaction due to the appearance of external electric field.It is usually shown only at the moment of chemical reaction, and its role is mostly to accelerate the reaction.[2]
Induced effect index
Quantitative scale of chemical activity.
Development History
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In 1923, GN. Lewis first proposed that the role of substituent groups in attracting or releasing electrons can cause valence on each bond in the whole moleculeElectron pairThe concept of movement.C. K. Ingrid et al. arranged common groups and atoms into a qualitative sequence with strong or weak inductive effects.
Based on a large number of experimental results in the physical and chemical properties, reaction equilibrium constants and rate constants of compounds, scientists have proposed various group characteristic constants to quantitatively or semi quantitatively express the strength of the induced effect of groups.The most famous one is LP. Hamilton's substitution constant σ (see Hamilton equation) and RW. Taft's polarity substitution constant σ *.
In 1962, Jiang Mingqian and Dai Cuichen believed that the induction effect was not only related to the electronegativity of bonding atoms, but also related to the bond length. According to the electronegativity of atoms and their covalent radiusInduced effect indexTo quantitatively express the strength of the induced effect of the group.
Comparison criteria
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The magnitude and direction of the induced effect are related to the electronegativity of atoms or atoms.When comparing the induction effects of various atoms or atomic clusters, the hydrogen atom in the C-H bond is usually used as the standardΙ=0。[3]
Atoms or atomic clusters (such as - X, - OH, - NO) with stronger electron attraction (greater electronegativity) than hydrogen atomstwo, - CN, etc.) have the induced effect of electron absorption or electron pulling (-IEffect), the electron cloud of the whole molecule is biased towards substituents.Atoms or atomic clusters with weaker electron attraction ability than hydrogen atoms have electron giving induction effect or electron pushing induction effect (+IEffect), the electron cloud of the whole molecule deviates from the substituent.
The induction effect of substituents has the following rules:
1. In the same group elements, the larger the atomic number is, the weaker the electron absorption induction effect is;In the same period elements, the larger the atomic number, the stronger the electron absorption induction effect.
2. GroupUnsaturatedThe greater the degree, the stronger the electron absorption induction effect.This is due to the fact thatsCaused by different orbital components,sThe higher the composition, the stronger the electron absorption ability.
3. The positively charged group and the group containing the coordination bond (directly connected) have the electron attracting induction effect, and the negatively charged group has the electron giving induction effect.
Figure 2 shows the order of the induced effect size of some atoms or groups
Electron withdrawing group: - NOtwo＞—CN＞F＞Cl＞Br＞I＞—C≡CH＞—OCHthree＞—CsixHfive＞—CH=CHtwo＞H。
Electron repelling group: (CHthree)threeC—＞(CHthree)twoCH—＞CHthreeCHtwo—＞CHthree—＞H。
The induction effect has a great influence on the acidity of organic acids.Any induction effect that can cause the decrease of electron cloud density on carbonyl carbon atoms can enhance the acidity, and the induction effect that can cause the increase of electron cloud density on carbonyl carbon atoms can weaken the acidity.The greater the impact, the greater the change in acidity.
For example, after the chlorine atom replaces the α - H of acetic acid, chloroacetic acid is generated. Because the - I effect of chlorine is transmitted through the carbon chain, the polarity of the O-H bond in the carboxyl group is increased, and hydrogen is easier to dissociate in the form of protons, thus increasing the acidity, resulting in ClCHtwoCOOH is more acidic than CHthreeAcidity of COOH.
Illustration
Descending rate
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The induction effect is a kind of electrostatic induction, and its effect decreases rapidly with the increase of distance.When the induction effect is transferred in a σ bond system, it is generally believed that every atom on the chain passes through, it will be reduced to about one-third of the original.The descending rate is usually expressed in ε or 1/α.After n atoms, the induced effect is only the original (1+α) n.It is generally believed that the induction effect can be ignored after three atoms.
Related extensions
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Conjugate effect
Conjugated systems originally refer to systems such as 1,3-butadiene with alternating single and double bonds, and generally refer to systems that can overlap with the side of π electron cloud and form polyatomic orbitals.The conjugate system produces the conjugate effect, and usesCexpress.If the substituent makes the π electron cloud density of the system increase, the substituent has the conjugation effect of electron donorCexpress;If the substituent reduces the π electron cloud density of the system, the substituent has the conjugation effect of electron absorption－Cexpress.
The strength of the conjugate effect is determined as:
1. The influence of atomic electronegativity.The greater the atomic electronegativity is, the stronger the electron absorption ability is, and the weaker the conjugation effect for electrons is.
2. The greater the degree of unsaturation of the system, the larger the conjugate system, and thus the greater the conjugate effect.
3. The influence of the charge carried by the substituent.The more negative charge, the stronger the conjugation effect of electron donor;The more positive charge, the stronger the conjugation effect of electron absorption;In general, the conjugation effect of negative charge electrons is greater than that of neutral molecules.
4. The influence of atomic energy level difference.The smaller the difference of atomic energy levels, the stronger the conjugation effect.[4]
Competitive relationship
Through the analysis of inductive effect and conjugate effect, the competitive relationship between them is summarized, as shown in Figure 3:
Figure 3
As can be seen from Figure 3, from the left side, the result is electron absorption effect, and there are the following cases:
(1) There is only inductive effect but no conjugate effect.Such substituents are more electronegative than carbon, and the first carbon atom connected to the substituent in the parent is saturated.
(2) The substituent is the group whose conjugation effect of electron donor is less than the induction effect of electron acceptor (+C＜－I）。If the adjacent carbon chain or ring of the halogen substituent is not saturated, the induction effect of the halogen electron absorption is greater than the conjugation effect of the donor electron, and the overall performance is electron absorption effect.
(3) The substituent is a group that has both the conjugation effect of electron absorption and the induction effect of electron absorption（－C+－I）。Such groups are usually positively charged groups or substituents with unsaturated bonds between the first carbon and non carbon atoms.
Seen from the right, the result is the electron donating effect.
(1) There is only conjugate effect but no inductive effect.This is the hyperconjugation effect. The order of magnitude is (CHthree)threeC-＞(CHthree)twoCH-＞CHthreeCHtwo-。
(2) The substituent is a simple group with lone pair electrons at other end groups except halogen, which can produce р - π conjugation.Such substituents are mostly groups or negative ions with lone pair electrons, such as amino, alkoxy, oxyanion andCarbonium anionEtc.
(3) The substituents are some complex groups that can produce р - π conjugation. Most of these substituents are complex. The group generally contains atoms of lone pair electrons and carbon carbon unsaturated bonds, forming р - π conjugation.It may be either electron absorption effect or electron giving effect.Usually some bivalent conjugated system groups.
application
In general:
1. The more electron deficient the system, the stronger the acidity;The more electron rich the system, the stronger the alkalinity.
2. The more electrons, the more stable the electron deficient system is, and the electron donor group can make the system more stable;The electron rich system is more stable with fewer electrons, and the electron absorbing group can make the system more stable.
3. The more stable the substance is, the easier it is to form.In the reaction process, stable intermediates are generated most and the reaction rate is the fastest. Some substances may generate more stable intermediates through rearrangement;The final product of the reaction is mainly the most stable substance.
give an example: Compare the acidity of the following compounds and rank them.
Analysis: The ortho isomer of aromatic ring carboxylic acid is the most acidic because:
(1) Induction effect.During ortho substitution, the distance between carboxyl group and nitro group is too close, which makes it difficult for the π electron on the carboxyl carbon oxygen double bond to conjugate with the π electron on the benzene ring, so induction is the main method.The substituent group attracts the electron cloud on the carboxyl group, thus increasing the dissociation of acid;
(2) Conjugation effect.The induction effect of meta position and para position is very weak, mainly conjugate effect, and meta position conjugate effect is greater than para position.The conjugation effect makes the electron cloud density on the carbonyl group increase, and the attraction to the carboxyl group decreases, thus weakening the acid dissociation shown.[4]
