Electromeric Effect (Bond Polarization) Homework Help, Tutoring
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Hyperconjugation (Bond Polarization) Assignment Help, Tutor Help
Bond Polarization - Temporary Effects
Unlike mesomeric and the inductive effects that are seen in compounds irrespective
of the presence of the attacking agent, there are certain temporary effects that
act only in the presence of the reacting agent. These effects include the
electromeric effect and the hyperconjugation.
Electromeric Effect:
Electromeric effect can be defined as the transfer of electrons from a double
bond or an atom with a lone pair of electrons to an adjacent single bond.
π electrons are involved in the transfer process. This involves a complete
transfer of electrons. Hence, complete negative and positive charges are
formed on the molecule. However, this effect remains as long as the attacking
molecule is present. The effect disappears as the attacking agent is removed.
This effect can best be represented as follows.
Reagent
C=C ------ -----→ C +-C -
→ ←-----------
C=C ------ -----→ C +-C -
→ ←-----------
No reagent
As can be seen above, the transfer of electrons takes place
towards a more electronegative atom. The atom obtaining the
electrons attains a complete negative charge and the one that
has lost the electrons develops a positive charge.
Like other effects electromeric effect represented by the letter E
exists in two forms the +E effect and the -E effect. The +E effect
is said to occur if the electrons are transferred towards the atom.
The -E effect involves displacement of electrons away from the atom.
As the electromeric effect is seen only in the presence of an attacking
agent, this effect is said to promote the reactions rather than inhibiting
them. When the inductive effect and the electromeric effect operate in the
same molecule, electromeric effect dominates the inductive effect. The most
common attacking reagents that initiate the electromeric effect are the
polar reagents. When these reagents are added to either C=C or C=O the
displacement of the π electrons takes place.
Hyperconjugation:
Unlike the electromeric effect which involves the transfer of π electrons,
hyperconjugation involves the displacement of both the sigma and π electrons.
This type of effect is mainly seen in atoms with a hydrogen atom on the α-carbon
atom attached to the unsaturated carbon atom i.e. (H-C-C=C). The extent of electron
displacement depends on the number of H atoms attached to the α-carbon atom. Greater
the number of hydrogen atoms attached to the carbon atom greater will be the electron
release. Thus, methyl with three hydrogen atoms shows a greater effect when compared
to ethyl with only two such hydrogen atoms.
Stability of the compounds such as alkenes can best be explained with the help of
hyperconjugation. For example 2-methyl propene with six C-H bonds is more stable
than propylene.
Solved problems
- Methyl amine is considered stronger than ammonia. This is due to which of the following effects.
- Electromeric effect
- Inductive effect
- Hyperconjugation
- Mesomeric effect
- The set of structures that represent the true structure of a molecule
but none of them alone can represent the molecule are called as
- Resonating structures
- Canonical structures
- Different structures cannot combine to explain a single molecule
- A and b
- Which of the following is a permanent effect
- Inductive effect
- Electromeric effect
- Hyperconjugation
- All
- The reactivity of alkyl halides and the strength of
carboxylic acids can best be explained with which of the following bond polarization effects.
- Electromeric effect
- Inductive effect
- Mesomeric effect
- Hyperconjugation
- Which of the following effects are seen in unsaturated molecules
with conjugative system?
- Inductive effect
- Mesomeric effect
- Electromeric effect
- Hyperconjugation
- Why are the electromeric and hyperconjugation effects termed as temporary effects?
- As they occur only in the presence of a attacking agent and disappear as the attacking agent is removed
- They exist only for certain time period.
- They are not temporary effects but are permanent
- none
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