Electronic Properties Flashcards
what is a conjugated double bond
single (sigma) and double (pi) bonds alternate which leads to delocalization of electrons
(belong to a region/group of atoms not a single one)
what does the delocalization of electrons lead to
increase in stability of compound
- due to similar bond length between sigma and pi bonds
What does the decrease in symmetry of MO mean?
higher energy
What are resonance structures
the alternating structures of a compound (that it can exist in)
What are the rules for generating resonance structures
- movement of electron pairs only (between structures)
- the less charged, the more favourable
- no similar charges on adjacent atoms
- negative charges on electronegative atoms favoured
- delocalization max in planar system possible
can the bonds be different to each other in a carbon ring?
(unless there are double bonds) the bonds are all identical in a ring
Draw the resonance structures of benzene
What are two allotropes carbon
graphite and diamond
Why is the Benzene structure from Hückels proposal so stable?
they have overlapping p orbitals. This means that there are dense electron clouds above and below the horizontal axis
Do orbitals alway have to have electrons in them?
no. they can be vacant
Where can electrons be found in an orbital?
anywhere except in a node. (in p orbital one electron can be on either side but not in the middle
are orbitals interrelated?
No. ther are superimposed. 1s orbital is not middle part of 2s for example. (they are separate)
When is a compound considered aromatic?
when it follows Hückel rules:
- # of pi electrons has to be 4N+2 (N=0,1,2,..)
- molecule is planar
- conjugated pi system
- possible charges have tobe delocalized
What is tautomerism?
(constitutional isomers)
An reaction that simply. involes the intramolecular transfer of a protein
real fast equilibrium between two isomers
name and draw 3 examples of an aromatic compound
give 2 examples of a tautomeric pair
what is an inductive effect (I)?
polarization of bonds due to different electronegativity (short range effect)
When do we haven a +I Effect?
- electron donation
- increase of electron density
- stabilization of electron deficient adjacent centres possible
What is an -I Effect?
- electron withdrawel
- decrease of electron density to stabilize electron rich adjacent centres
What are examples of electron withdrawing groups (for inductive effect)?
- NO2
- F
- COOH
- Cl
- Br
- I
- OH
What are examples of electron donating groups (inductive effect)?
- Me3C
- Me2CH
- MeCH2
What is a Resonance (mesomeric) Effect?
delocalization of lone pairs of pi bonds between resonance structures
only formal migration of lone pairs and bonds helps visualize densities
What is a +M Effect?
donation of lone pair
- stabilization of electron deficient center (increase electron density)
What is the -M effect?
removal of electron pair
- stabilizing electron rich centers
What are examples of electron accepting groups (-M)
- COOH
- COOR
- CN
- CHO
What are electron pushing groups (+M)?
- Cl
- Br
- I
- OH
- OR
what is Hammets reaction constant ρ?
is a measure of reactions’s sensitivity to substituent effect
what does a positive rho mean?
more electrons in transition state than in starting material (electron withdrawing group aids)
What is hyperconjugation?
stabilization interaction that results from the interaction of electrons in filed orbital (usally in sigma bond) with adjacent empty or partially filled orbital to give an extended molecular orbital that increases stability of the system
