Inductive effect

The inductive effect or I-effect is a charge-changing effect in organic chemistry, which occurs both as a + I-effect ("electron- pushing ") and as a −I-effect ("electron drawing"). It is triggered by electrostatic induction by functional groups along one or more chemical bonds . The concept was developed by chemists Gilbert Newton Lewis and Christopher Kelk Ingold .

basis

The cause of these effects is an asymmetry in the distribution of electrons in an electron pair bond between two identical but differently substituted atoms or between two different atoms. Two atoms bound by an electron pair bond share two electrons. These electrons are not assigned a fixed place, but are freely movable within this bond. In the case of an asymmetry of the electron distribution, the electrons are attracted to the atom whose electronegativity is greater.

There are two types of I-effects: the + I-effect (read: positive inductive effect) and the −I-effect (read: negative inductive effect). A more electronegative atom has an −I effect, so that the electron density of the other atom is reduced. With a + I effect, the electrons are pushed away from one atom and thus the electron density on the other atom increases. The bond now has a dipole character , which is characterized by δ ⁺ on the atom with the lower charge density and δ ^- on the atom with the high charge density. To compare the strength of the inductive effect of atoms or groups of atoms, the electronegativity of the substituent is compared with the electronegativity of hydrogen . The greater the difference in electronegativities, the stronger the inductive effect.

General

Covalent bonds can, depending on the electronegativity of the bond partners, be polarized, i.e. they can be present as polar atomic bonds . If one of the elements is more electronegative than its binding partner, the electrons are more often in its vicinity. This shifts the charge distribution so that the more electronegative element is more or less strongly negatively polarized. There is a bond dipole moment along the bond.

An example is water (H ₂ O). Due to the higher electronegativity, the electrons keep frequent in oxygen - Atom on. In the water molecule, this is expressed by δ ^- near the O atom, as well as by a δ ⁺ next to each of the two H atoms. Often the δ ^- with oxygen is written a little larger. This is common because the δ ^- charge on the oxygen atom is twice that of each individual hydrogen atom. Vector addition of the individual bond dipole moments results in the electrical dipole moment of the molecule. Bonding dipole moments and electrical dipole moments should not be confused with one another: For example, symmetrically built molecules such as B. carbon dioxide (O = C = O) polar bonds, but no electrical dipole moment.

Bonding dipole moments of the water molecule	Binding dipole moments and electrical dipole moment of an H 2 O molecule. In green: electric dipole moment . ${\displaystyle {\vec {p}}}$

The inductive effect can affect other atoms or groups of atoms across multiple bonds. However, the strength decreases with the square of the distance. It is assumed that inductive effects have no further effect than three adjacent bonds. If an inductive effect occurs in a molecule such as 1-fluoropropane , the inductive force also acts on the following atoms in a chain:

The fluorine atom triggers an induction effect that affects the three following carbon atoms. The induction effect is strongest on the first carbon atom, which is bonded directly to the fluorine atom, this is indicated by the symbol δ +. However, the strength decreases the further the affected carbon atom is from the fluorine atom. The induction effect on the second carbon atom in the alkyl chain is lower, which is expressed by the δδ + mark. Again, the inductive effect of the fluorine atom has a much smaller effect on the third carbon atom, which is even further away from the fluorine atom, which is expressed by the marking δδδ +.

As a rule, I-effects are considered for more complex connections. This makes it possible to analyze the behavior of the connections. For example, the −I effect with trichloroacetic acid has more extensive effects. In this connection, three Cl atoms on the C atom have an −I effect. As a result, the carbon atom attracts the electrons of the neighboring carbon atom, whereby this carbon atom attracts electrons from the neighboring and simply bound oxygen . The bond between the O atom and the H atom connected to it is weakened and the H ⁺ ion (proton) can be split off very easily.

+ I effect

+ I effect of the lithium atom in methyllithium

Particles that have an electron-pushing effect have a + I effect. This happens e.g. B. when the particle is negatively charged or has a low electronegativity. The + I effect can also be observed in the formation of hybrid orbitals . B. the methyl group CH ₃ electron-donating, even if this cannot be seen due to the C – C single bond.

−I effect

−I effect of the bromine atom in bromomethane

Atoms that have an electron-withdrawing effect have the −I effect. This is usually due to high electronegativity or a positive charge. Strongly electronegative particles attract electrons particularly strongly.

Effects of the induction effect

The effects of the induction effect are that other polar molecules can now align themselves with the said molecule and attack it. In addition, the induction effect has an influence on the position of the second substituents on aromatic systems. Radicals or carbenium ions (carbocations), i.e. particles with a deficiency of electrons, are stabilized by substituents with a + I effect and destabilized by those with a -I effect. Apart from that, the inductive effect has a decisive influence on the acidity of a molecule. For example, if a molecule has a strongly electronegative (electron-attracting) substituent, the splitting off of a proton is facilitated (−I effect) and the acid strength is correspondingly high. Conversely, an electron-donating substituent leads to a low acid strength (+ I effect).

The + I effect also has an influence on the position of the second substituent in electrophilic aromatic substitution . A first substituent with a + I effect pushes electrons into the system. However, the inductive effect quickly decreases with distance. If substituents are bound in the ortho or para position, a cation is created as a transition state, which can be represented in mesomeric boundary structures. There is always a boundary structure in which the positive charge of the cation is at the carbon atom that has bound the first substituent. Because the electrons pushed into this boundary structure are right next to the positive charge, the cation is stabilized. The more stable transition states in the ortho or para position have a relatively low energy and are therefore reached relatively quickly (see Hammond postulate ). It is also said that primary substituents with a + I effect direct in the ortho or para position. The para position is preferred to the ortho position because of less steric hindrance .

Inductively acting groups

Here are some inductive groups listed:

+ I (positive inductive effect)

• t - butyl group -C (CH ₃ ) ₃
• i - propyl group -CH (CH ₃ ) ₂
• Alkyl radical -R

I = 0 (no inductive effect)

• Hydrogen atom -H

−I (negative inductive effect)

• Oxygen in the carbonyl group –C = O
• Hydroxy group -OH
• Iodine atom -I
• Bromatom –Br
• Chlorine atom -Cl
• Nitro group -NO ₂
• Amino group -NH ₂
• Carboxy group -COOH
• Fluorine atom -F
• Cyano group -CN

See also

• Mesomeric effect

Individual evidence

1. ↑ Entry on inductive effect . In: IUPAC Compendium of Chemical Terminology (the “Gold Book”) . doi : 10.1351 / goldbook.I03021 .
2. ^ K. Peter C. Vollhart, Neil E. Shore: Organische Chemie , Wiley-VCH Verlag, 5th edition (2011), pp. 787ff., ISBN 978-3527327546 .
3. ↑ M. Liersch: Chemie 2 Kurz und Klar , Auer 1996.