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RESONANCE AND INDUCTION TUTORIAL
Jack DeRuiter
The terms "resonance" and "induction" refer to the electronic effects that atoms or functional groups may have within a compound. These effects are defined below and are dependent on the valence, bonding order and electronegativity of atoms, as well as the molecular geometry. Thus it is important that you understand these concepts prior to reviewing this tutorial.
I. INDUCTION
Induction or the inductive effect of an atom or functional group is a function of that groups 1). electronegativity, 2). bonding order and charge and 3). position within a structure. Inductive effects refer to those electronic effects of an atom or functional group can contribute through single bonds such as saturated (sp^3 ) carbon atoms! This is very different from resonance effects discussed later in this section. The contribution of electronegativity, bonding order and position toward induction is as follows:
Electronegativity: Atoms or functional groups that are electronegative relative to hydrogen such as the halogens, oxygen, nitrogen, etc. may have a negative inductive effect (-I), depending on their bonding order (see the Table below). Thus these atoms withdraw electron density through the single bond structure of a compound and can assist in the stabilization of negative charge that may form in reactions. One such reaction where -I groups can have a stabilizing (enhancing) effect is the ionization of acids. Consider the case of acetic acid, chloroacetic acid and trichloroacetic acid shown below. All three of these compounds can ionize (loss of proton from the carboxyl OH). The only difference between these three structures in the degree of chloro group substitution. Chlorine atoms are electronegative (three pairs of non-bonded electrons in their valence shell) and thus have a -I effect. Thus they can help stabilize a negative charge, and enhance the ionization of an acid. Note the pKa differences between acetic acid and chloroacetic acid. Furthermore, the more Cl atoms present, the greater the total -I effect and the greater the ease of ionization (lower pKa). Again, note that the electronic effect in this example is being "induced" through single bonds; a saturated (sp^3 hybridized) carbon in this case.
Atoms or functional groups that are electron donating (hydrocarbons, anions) have a positive inductive effect (+I). These groups can help stabilize positive charges in reactions such as protonation of bases.
H C
H
H
O
OH
Cl C
H
H
O
OH
Cl C
Cl
Cl
O
OH
Acetic acid Chloroacetic acid Trichloroacetic acid
pKa = 4.76 pKa = 2.87 pKa = 0.
- Bonding order and charge: As mentioned above, it is important to consider both the electronegativity and bonding order when analyzing the inductive potential of an atom. For example, oxygen in a hydroxyl group (OH) is electron withdrawing by induction (-I) because the oxygen atom is relatively electronegative and is uncharged in that bonding arrangement. However, oxygen in an "alkoxide" (O-) structure is electron donating (+I) by induction because in this bonding order (a single bond to oxygen) it has an "excess" of electron density. Thus an OH group would help to stabilize a negative charge within a structure, while it's ionized form, the alkoxide, would stabilize a positive charge!
- Bonding position: The strength of the inductive effect produced by a particular atom or functional group is dependent on it's position within a structure. For example, the further from the site of ionization, the lower the inductive effect. This is illustrated in the example below where the acid with the chlorine atom positioned on a carbon atom nearer the reaction site (OH) is more acidic that the acid where the chlorine atom is positioned further away:
It is also important to understand the difference between inductive effects and resonance effects of a particular atom or functional group and the relationship to bonding position. For example, an oxygen atom in a hydroxy group (OH) is electron withdrawing by induction, but electron donating by resonance when placed in a position on the structure where resonance is possible This will be explained more fully below.
II. RESONANCE
Resonance may be defined as bonding or sharing of electrons between more than two atoms (nuclei). Typical covalent and ionic bonding involves sharing (covalent) or transferring (ionic) electron pairs between two atoms as shown in the examples of ethane and sodium chloride below. In these examples the bonding electrons are localized :
C
H
H
O
OH
H
Cl H
C
H
Cl
O
OH
H
H
H
More acidic Cl "closer" to the reaction site Cl "further" from the reaction site
Less acidic
H C C H
H
H
H
H
Na+^ Cl
Ethane Sodium Chloride
Localized covalent bonds Localized ionic bond
can be illustrated by butadiene. In this example the resonance structure shows that the electrons can be "shared" or delocalized (follow the arrows) creating a cationic and anionic center:
Another example of the single bond-double bond resonance pattern is in α, β-unsaturated carbonyl compounds. Here the difference in electronegativity between atoms in the pattern allows for a dipole and reactivity such as conjugate addition:
Examples of resonance resulting from atom with non-bonded electron pairs bound to a double bond system are shown below. Notice in each case below that that the "resonance phenomenon" involves an electron rich atom donating a pair of electrons to an electron deficient multiple bond system, and that in actually, the charges shown are distributed or delocalized over all of the atoms of conjugation. This resonance phenomenon is what makes carboxylic acids, and to a lesser extent phenols and protons alpha- to a carbonyl, acidic! The charged formed upon ionization can be stabilized through resonance delocalization!
H
H
H
O
H
H
H
H
H
O
Resonance structure
R O -
O
O
R H
R
R
O -
O
R H
R
O - (-)
O -^ O
(-)
ETC
H
H
H
H
H H
H
H
H
H
H
H
Butadiene Resonance structure
Resonance can also occur in structural patterns where an atom deficient in an electron pair is bound to a double bonded system. Consider the case of the reaction of the "allyl chloride" compound below. This compound can ionize by loss of Cl-^ (a good leaving group) because the charge of the resulting carbocation is stabilized by resonance delocalization of the pi (double bond) electrons. The same is true for benzyl alcohols as shown below. Benzyl alcohols can ionize because the resulting carbocation charge can be delocalized throughout the benzene ring via resonance:
In every case above, resonance was possible because of an electronic configuration in which there is an alternating single bond-double (pi) bond pattern, or an atom with non-bonded electron pairs (or lacking an electron pair) bound to a double bond system. It is important to realize that resonance is not possible when such an electronic configuration is NOT possible. Consider the examples below. In the first example (1) resonance is not possible because the two double bonds are separated by MORE THAN on single bond. In the second example the negatively charged atom is separated from the double bond by more than one single bond and in example (3) the charge is separated from the C=C by more than a single double bond.
H
CH 3 CH 3 Cl Allyl chloride
H
CH 3 CH 3 (+)
H
CH 3 CH 3
Cl (+)
R
OH
R
(+) R (+)
ETC
Benzyl Alcohol
Carbocation Resonance Stabilization
Carbocation (^) Resonance Stabilization
H
CH 3
H
H (1)
H
H
O -
CH 3 (+)
(2) (^) (3)
NO RESONANCE POSSIBLE!!
- Electron donating (+R) and Withdrawing (-R) Groups: electron donating and withdrawing groups by resonance are listed in the table on a previous page. Note that electron donors (+R) have at least one pair of non-bonded electrons on the atom involved in resonance (OH, OR, NRR, SR, etc.). As a result, +R groups can facilitate reactions that involve the formation of cations, and enhance the basicity of amines. This is illustrated in the examples below:
- Electron withdrawing groups (-R) are characterized by electron deficient atoms linked to a site of conjugation. As a result, -R groups can facilitate reactions that involve the formation of anions, and enhance the acidic of acids. This is illustrated in the examples below:
III. RESONANCE VERSUS INDUCTION:
As described above, induction involves the electronic effects of atoms and functional groups through saturated carbons, and are dependent on bond valence and position. Resonance involves the sharing or delocalization of electron pairs over more than two atoms and requires conjugation and coplanarity. To compare and contrast these two electronic effects,
HO
OH
CH 3
HO
CH 3
(+)
HO
CH 3 (+)
OH +R stabilization of a carbocation
O
NH 2
CH 3
. O
CH 3
NH 2
H+
H (^) H
O
R
R
O
H
O
R
R
O
(-) and
(-)
O
R
R
O
H
N
O
O H
(+) (^) (+) N
O
O -^ (+) N
consider the electronic effects of the hydroxy group (OH). This group is a withdrawer by induction (-I) and an electron donor by resonance (+R). Thus when positioned within a structure where it can participate in delocalization of pi electrons, it will function as a strong electron donor. When placed in a structure where its resonance effects are "insulated" by single bonds, only its electron withdrawing inductive effect will be felt.
C
H H H H^ C^ H H C^ H
Fully saturated carbon with no NBEs. R not possible! +I Effect
Loss of H +^ : Carbanion: NBE pair available for donation: +R Effect
Loss of H - Carbocation: Vacant orbital ("short" a NBE) -R Effect
CARBON SUBSTITUENTS AND ELECTRONIC EFFECTS
H C
C
H H
+R -R
Alkene carbon: Can donate (+R) or withdraw (-R) electron density
C
O H C
O CH 3 C
O OCH 3 C
O OH
Carbonyl-containing functional groups (aldehydes, ketones, ester, acids) in conjugation with an aromatic ring (or other site of potential resonance) can withdraw electron density by resonance (-R) as shown above. These functional groups also withdraw electron density weakly by induction. The same is true, to a lesser degree, for analogues of the compounds above where the carbonyl oxygen is replaced by an atom of the same group (sulfur), or multiple bonded atom of the same row (as in nitrogen shown below)
C R N^ R C N
Aldehydes Ketones (^) Esters Carboxylic Acids
Imines Nitriles Unsaturated carbon-nitrogen functionality (imines and nitriles and similar structures) in conjugation with an aromatic system, like the carbonyl-containing functional groups can also withdraw electron density by resonance (-R) as shown above. These functional groups also withdraw electron density weakly by induction.
The electronic effects of an atom are a function of ionization and hybridization state as well as the nature of the structure to which they are linked (are they "in conjugation" with the structure to which they are linked):
The electronic effects of an atoms are determined or modified by the nature of the functionality in which they are incorporated:
NH 2
NHCH 3 N(CH 3 ) 2 CH 3 N CH 3 "
CH 3
Primary Aniline (+R)
Secondary Aniline (+R) Tertiary Aniline (+R)^ Quaternary Aniline (-I)
Loss of H +
NH
Anion (Strong +R)
THE ELECTRONIC EFFECTS OF NITROGEN SUBSTITUENTS
NH
O
CH 3
Anilide (Amides): Weak +R due to"competing resonance" through the bonded carbonyl
N
O
Nitroso: Electron Withdrawing (-R) N in conjugation with electronegative oxygen
N O
O
Nitro: electron Withdrawing (-R) N in conjugation with two electronegative oxygens
Protonation
Anilinium Ion (-I): NBEs no longer available!
NH 3
Primary Aniline in conjugation (+R)
NH 2 NH 2
Primary Amine NOT in conjugation (-I)
CH 2 NH 2
Primary Amine NOT in conjugation (-I): sp 3 CH 2 group "blocks" resonance contribution
Electronic effects of an atom determined by nature of bonding and potential for conjugation:
Electronic effects of an atom determined by the ionization state:
Alkyl substitution does not change the nature of potential electronic effects of an atom, unless alkyl substitution changes the bonding order (i.e. "occupies" nitrogen's lone pair of NBEs):
The electronic effects of an atoms are determined or modified by the nature of the functionality in which they are incorporated:
Principles of Relative Acidity
- Acidity: Abililty of an atom to "give up" a proton and ionize (or ability of an atom to "accept" an electron Dependent on strength of X-H bond (see below) and stability of conjugate base (X-^ ):
R X H Potential Acid (X= N, P, S, halogen, etc.)
R X
Bond Strength
Ion Stability
Conjugate Base
- Relative acidities of functional groups with a "common" heteroatom (X): Example for Oxygen
When comparing functional groups with a common heteroatom, relative acidity is primarily a function o stability of the conjugate base. In the examples below, ALL of the functional groups have a common O- acidic fragment, and the O-H bonds strengths are roughly equivalent. But yet these compounds differ si cantly in acidity because of differences in the stability of the conjugate bases. Note that those O-H funct groups that are most acidic are those in which the O -^ charge of the conjugate base is delocalized over mo electronegative atoms (the R group is the same for all functional groups):
R S
O
O
O H
R C
O O H
O H
R O R H Sulfonic Acid (Charge over 3Os and S)
Carboxylic Acid (Charge over 2Os and C)
Phenol (Charge over 1O and 6Cs)
Alcohol (Charge over 1O only!)
- Relative acidities of compounds with common functional groups but different substituents: Substituent E Compounds containing the same acidic functional group by different substituent groups may differ in the acidities based on the potential differences in electronic effects of different substitutents (see the "Resonance and Induction" chapter). For example, groups that withdraw electron density by induction or resonance will enhance acidity by stabilizing the negativecharged formed in the conjugate base. Conv those that donate electron density with decrease acidity. Consider the examples below:
Cl C
Cl
Cl
C
O
O H
Cl C
H
H
C
O
O H
H C
H
H
C
O
O H
Acids and an Electron Withdrawing Cl group(-I)
O H
NO (^2)
Phenols and Electron Withdrawing & Donating G groups by R
O H
H
O H
OCH 3
-R Group Electro-Neutral Group
+R Group
Methane (Very, Very weak acid)
Ammonia (Very Weak Acid)
Water ("Weak Acid")
Hydrogen fluoride ("Strong acid")
H C H H
H H N H H
H F H O H
- Relative acidities of functional groups with "different" heteroatoms (X) with a common valence shell: The Electronegativity rationalization:
The valence electrons of atoms in the same horizontal row of the periodic table (i.e. Li, Be, B, C, N, O, F all exist in the same valence orbital (the 2s-2p orbitals for the row two atoms listed). The primary differe between the atoms of a row then is HOW MANY electrons are in the valence shell and how "electronega the atom is. As the number of valence electrons IN A COMMON VALENCE ORBITAL increases, the electronegativity increases. Thus the electronegativity trend for the row 2 atoms is F>O>N>C>B etc. Within this row of atoms, the higher the electronegativity, the greater the ability to stabilize negative char as forms during acid ionization. It should be noted that the strength of the heteroatom-hydrogen (X-H) b also increases with the number of valence electrons in a row, but this effect is less important than the stab effect on the negative charge formed upon ionization. Thus relative acidities for functional groups with d heteroatoms of the same row (valence shell) parallels elctronegativity as illustrated below:
Example: (^) R C
O O H
R C
O N H
H
Carboxylic Acid (^) Primary Amide
- Relative acidities of functional groups with "different" heteroatoms (X) and a different valence shell: The "Polarizability" rationalization:
The valence electrons of atoms in the same group (column) of the periodic table (i.e. O, S, Se, Te, etc.) exist in increasing higher valence orbitals (i.e the 2s-2p for oxygen, the 3s-3p for S, etc). The primary difference between atoms (other than the obvious, the number of protons, neutrons and electrons) of the same group (column) is how far from the nucleus the valence shell is located (one shell further out for S t O). The further the valence electrons are from the nucleus, the greater the space or area over which they distributed. If charge forms in the valence shell, as in the ionization of an acid, the further this shell is fro the nucleus, the greater the area for charge distribution and the more stable the charge. Also, the further t valence shell is from the nucleus, the weaker the heteroatom-hydrogen (X-H) bond. For these reasons, atoms "further down" in a group of the periodic table are better able to stabilize charge and will therefore ionize more readily as illustrated by the examples below:
H
Se H H^
S H H^
O H
S H^ O H
Example:
Conjugate Base
O H
Phenol (pKa 10)
OH
RELATIVE ACIDITIES OF PHENOLS: SUBSTITUENT EFFECTS
OH
R
Alkyl substitution at positions 2, 3 or 4 decreases acidity modestly due to the +I effect
Halogen (F, Cl, Br, I) substitution at positions 2, 3 or 4 increases acidity modestly as a result of the -I effect of halogen (F>Cl> etc.)
X
OH
Substitution with electron donating groups by resonance (+R) at positions 2 or 4 (in conjugation) decreases acidity significantly (- charge destabilization). D-R Examples: -OCH 3 , -SCH 3 , NHCH (^3)
D R
OH (^) OH
W R
Substitution with electron withdrawing groups by resonance (-R) at positions 2 or 4 (in conjugation) increases acidity significantly (- charge stabilization). W-R Examples: -NO (^) 2, -COCH3, -COOCH3, -CN, -SO (^) 2CH 3 , etc.
Example: O
OCH 3
O
C O CH^3
Example:
Electron donating group "destabil- izes developing negative charge in the base by +R
Electron withdrawing group "stabilizes" developing negative charge in the base by -R
pKas > 10
pKas < 10
pKas >> 10 pKas << 10 (i.e. 7 or lower)
(Be able to identify functional groups as electron donating or withdrawing by induction or resonance, and to draw resonance structure, when possible!)
PROBLEMS: RESONANCE AND INDUCTION (and related reactivity)
- Why are amides less acidic than carboxylic acids?
- Why are amides less susceptible to hydrolysis than esters?
- Answer the questions below (a-c) for the follow series of compounds:
A B C D E
a. Which compound above is MOST acidic? b. Which compound above is LEAST acidic? c. Which compound above is MOST susceptible to hydrolysis?
- Rank the following set of compounds in terms of relative ACIDITY (1 = most, 5 = least):
CH 3
CONH 2
CH 3
OH
CH 3
COOH
CH 3
CO 2 CH 3
CH 3
SO 2 NH 2
SO 3 H
NO 2
CH 2 SH
NO 2
SH
NO 2
CH 2 OH
NO 2
OH
NO 2
- Explain the difference in reactivity between the two alcohols shown below:
- Identify the compound below that would be most likely to form an imine with a primary amine? Identify the compound below LEAST likely to form an imine:
NO REACTION
HCl CH 3 CH 2 OH
OH HCl Cl
NO 2
H
H
HO
OCH 3
O CH
3
OCH 3
O H
NO 2
O H
