General Review Flashcards
(1st set of PP slides)
Why do Atoms bond?
- each atom wants a full
outermost energy level - gain, lose, and share valence
electrons to achieve the duet or
octet rule aka: “being happy” - gives each atom an electron
configuration similar to that of a
noble gas
ex. Group 18: He, Ne, Ar, etc
Chemical Bonds
are attractive force that holds atoms or ions
together
- determines the structure of compound
- structure affects properties
- melting/boiling pts, conductivity etc.
Electronegativity
The tendency of an atom to attract
electrons from a neighboring atom.
Electronegativity value of 0.0 - 0.5
Covalent nonpolar
Electronegativity value 0.5-1.67
Covalent polar
Electronegativity value >1.67
Ionic
Dipole moment
lambda? (kinda like an n)= Qr
where Q is the magnitude of the charges and r is the distance
Ionic bond
When an atom of a nonmetal (becomes negative cation) takes one or more electrons
from an atom of a metal (become positive cation) so both atoms end up with
eight valence electrons.
When the electronegativity of two atoms
are quite different from each other:
One atom loses an electron (or
electrons). The other atom gains an electron (or
electrons).
-bond formed by the attraction between oppositely charged
ions
Properties of Ionic Compounds
Structure affects properties
- strong attractions between ions: strong bonds
- high melting/boiling pt
- shatter when struck (think of it as one unit)
- conductivity
solid: ions are so close together, fixed
positions, (can’t move)
NO conductivity
liquid: ions are freely moving due to a
broken lattice structure
Good conductivity
Covalent bonds
- chemical bond in which two atoms share a
pair of valence electrons - can be a single, double, or triple bond
single, 2e-’s (-); double, 4e-’s (=); triple, 6e-’s(Ξ) - always formed between nonmetals
- mostly low melting/boiling points
- 2 types of bonds
- polar
- non-polar
Co-ordinate Bond
- When one atom donates both electrons in a
covalent bond. - Carbon monoxide
- EX: CO
- Co-ordinate covalent bond is the above except in a covalent bond, ex’s include NH4+, H30+, etc
Hydrogen bonding
Are the attractive force caused by
hydrogen bonded to F, O, or N (via small dipe charges.)
* F, O, and N are highly electronegative,
so it is a very strong dipole.
* The hydrogen partially share with the
lone pair in the molecule next to it.
* The strongest of the intermolecular
forces.
Sigma Bond
A bond where the line of electron density
is concentrated symmetrically along the line connecting the
two atoms.
Pi bond
A bond where the overlapping regions exist
above and below the internuclear axis (with a nodal plane
along the internuclear axis).
Electronic effects
- Inductive effect
- Resonance or Mesomeric effect
- Electromeric effect
- Hydrogen bond
- Steric effect
- Hyper conjugation (Not required)
Localized electrons
restricted to a particular region
Delocalized electrons
electrons do not belong
to a single atom or exclusively to a
bond between 2 atoms
Inductive Effect
permanent displacement of shared
electron pair in a carbon chain towards more electronegative atom or
group.
Due to the difference in electronegativity between two atoms linked up by
sigma bonds, the bonding electrons will displace towards the more electronegative atom. The
atom exhibits a partial negative charge.
The electronic effect of a group that is transmitted by the polarization of electrons in
sigma bonds is called an inductive effect.
Permanent effect in saturated carbon chain compounds.
Group attached to carbon chain should have tendency to release or withdraw
electrons.
Negative Inductive Effect : (—I effect, Electron withdrawing
effect)
When an electronegative atom or group (more electro negative than hydrogen) is attached to the terminal of the carbon chain in a compound, the electrons are displaced in the direction of the attached atom or group.
Ex’s: —NO2 > —CN > —COOH > F > Cl > Br > I > OH > C6H5 > H
Positive Inductive effect : (+I effect, Electron
releasing/donating effect)
When an electro positive (actually, less electronegative) atom or group
(more electro positive than hydrogen)is attached to the terminal of the
carbon chain in a compound, the electrons are displaced away from
the attached atom or group.
(CH3)3C— > (CH3)2CH— > —C2H5 > —CH3
Inductive Effects and Carbocation Stability
The more methyl groups (-ch3’S) a carbon has attached to it, the more stable it is. So least stable is 3 H’s (not 4, main C with + charge), H2-C+-CH3 primary carbocation, H-C+-(CH3)2 secondary, and C+-(CH3)3 is from least stable to most stable. The methyl groups have +I inductive effects towards the center/main carbon to offset + charge. Or put another way, they are more stable because electron-donating groups exert positive inductive effects to reduce the positive charge on the (main) carbon atom.
Electromeric Effect
defined as the complete transfer of electrons of a multiple bond towards one of the bonded atoms at the demand of an attacking reagent. This phenomenon of movement of electrons from one atom to another at the demand of attacking reagent in multibonded atoms
is called electromeric effect, denoted as E effect. The electromeric shift of electrons takes place only at the moment of reaction.
-Temporary effect which is observed in presence of reagents involving transfer of electrons in an unsaturated system.
-It is shown by those compounds containing multiple bond
+E effect (types of electromeric effects)
When the electron pair moves towards the
attacking reagent, it is termed as the +E effect. The addition of
acids to alkenes is an example of the +E effect. After the transfer
takes place, the reagent gets attached to the atom where the
electrons have been transferred to, e.g : addition of H+ to alkene.
-E effect (types of electromeric effects)
When the electron pair moves away from the
attacking reagent, it is termed as the -E effect . e.g : addition of
cyanide ion (CN-) to the carbonyl group
Resonance
Each C is sp2 hybridized
* Each C has an unhybridized p
orbital perpendicular to the plane of
the ring
* The 6 p orbitals overlap to form a pi
cloud
Resonance Hybrid
Canonical Structures
– resonance contributors
– resonance structure (or hybrid)
EX: Benzene rings with opposite double bonds are resonance contributors, with hybridized structure with dashes all around is the resonance hybrid
For resonance hybrid structures:
1) Only e- move (not atoms)
* 2) Only pi and non-bonding e- move
* 3) Total # e- stays same (as does unpaired e-)
Resonance hybrid electron movement
-pi e- move toward + charge or toward pi/double
bond (in example on slide positive charge is conjugated with double bond, and so electrons can move)
-Nonbonding pair e- toward a pi bond
(in example on slide negative charge is conjugated with double bond)
- Nonbonding single e- toward a pi bond (in example on slide a single electron is conjugated with double bond)
What makes a Resonance Structure
Have Decreased Stability?
1) an atom with an incomplete octet
2) a negative charge that is not on the most
electronegative atom
3) a positive charge not on the most
electropositive (least electronegative) atom
4) charge separation
Mesomeric/ Resonance Effect
The flow of electrons from one part of a conjugated π system to the other caused by
phenomenon of resonance is called Resonance effect or Mesomeric effect.
Negative Mesomeric/Resonance effect (-M/E effect)
When the electron displacement is towards the group (EWG), e.g :-NO2 ,
-CHO
Positive Mesomeric/Resonance effect (+M/E effect)
When the electron displacement is away from the group (EDG), e.g :-
OH , -NH2, -OR, -Cl
Resonance Energy/Stability
- 1) The greater the predicted stability of a
resonance contributor, the more it
contributes to the resonance hybrid. - 2) The greater the number of relatively
stable resonance contributors, the greater
the resonance energy. - 3) The more nearly equivalent the resonance
contributors, the greater the resonance
energy.
