11 rules you must know to compare stability of resonating structures

Apply these rules in decreasing priority order, i.e first you should apply Rule 1, then Rule 2 and so on.

This slideshow requires JavaScript.

 RULE 1-AROMATICITY

dsc_0706

AROMATIC STABILISATION makes resonating structures most stable whereas ANTI-AROMATIC structures are least stable. for example, consider Cyclopropenone;
Here, in figure,  Cyclopropenyl Cationic system is Aromatic (being completely conjugated and having 2π electrons)  hence more stable.

RULE 2-COMPLETE OCTET

dsc_0683The structure having COMPLETE OCTET of all atoms (of 2nd period elements) or GREATER NUMBER OF COVALENT BONDS is more stable.(for hydrogen atom , duplet is needed for stability) (note- The  structure having expanded octet i.e, more than eight electrons in valencee shell around second-period elements e.g, Be, B, C, N, O, F is not possible) for example, consider diazomethane (CH2N2) – Here in structure B, all atoms have complete octet, hence more stable.

RULE 3-CHARGE SEPARATION

rule3Charge-separation increases energy of the system hence uncharged Structure (the structure having formal charges of each atom zero) is more stable than polar or charged (having non-zero formal charges of one or more atoms) structures.Here, in figure, II and III are very less contributing as these are charged structures, hence less stable.

RULE 4-BACK BONDINGdsc_0713

Those structures are more stable where Negative charges or lone pairs are closer to atoms having positive charges or vacant p or d – orbitals (e.g, Boron atom) an vice- versa.(these are stabilised by back-bonding or dative π-bonding formation) Here, in figure, Stability Order :  A > B > D > C

(note- Back bonding is significant only when atoms involved are of 2nd or 3rd period)(note- Tetravalent nitrogen carrying positive charge as in ammonium ion etc, cannot take part in back-bonding as they cannot expand their valence shell beyond octet) 

RULE 5- ELECTRONEGATIVITYcapture

The structure having negative charge on more electronegative atom and positive charge on more electropositive atom is more stable.(Note- this rule applies well only when atoms concerned are from same period or same atom having different hybridisations)

RULE 6-POLARISABILITYdsc_0694

Going down the group, negative charge is more stabilised, particularly useful in case of atoms of group 16 or group 17 (based on polarisability, larger anions have greater dispersion of charge over large surface area hence more stable) Here, in figure, Stability order: RO- < RS-

(note-going down the group, size factor dominates over electronegativity)

Subscribe Youtube Channel to Learn, Ask and Master chemistry concepts, tricks & Problems

RULE 7-ELECTRONIC EFFECTSdsc_0707

Inductive effect, hyperconjugation effect helps in further deciding stability of charges or structures. Here, in figure, Tertiary carbocation is more stable due to hyperconjugation as well as  +I effect.

RULE 8-HYBRIDISATIONdsc_0695

Negative charge is more favourable on sp hybridised atom whereas positive charge is more favourable on sphybridised atom.In Figure, structure A having negative charge on sp2 hybridised nitrogen is more stable than sp3 hybridised nirogen in B. (note-as “s” character of the orbital increases, electronegativity increases)

RULE 9-COULOMBIC FORCEnew

The structure having opposite charges closer and like charges farther is more stable.

“STABILITY ORDER: A > E > B > C > D “

RULE 10- BRIDGEHEAD POSITIONdsc_0711

Positive charges or free radicals or multiple bonds are not stable at bridgehead carbon atoms. (Bredt’s Rule- At Bridgehead position, planarity is difficult to attain unless rings are large enough.)

RULE 11- CROSS CONJUGATION

dsc_0708

Carbocation having linear conjugation is more stable than in case of cross-conjugation. CROSS CONJUGATION is less stabilising than LINEAR CONJUGATION.

SPECIAL EXAMPLE

dsc_0696

O2 is a diradical, having one sigma bond & two unpaired electrons (refer molecular orbital theory)



Practice Problem- 

Draw resonating structures of

(i) N2O (nitrous oxide) &

(ii) N3 (azide ion) and arrange them in stability order:



Advertisements

One thought on “11 rules you must know to compare stability of resonating structures

Leave a Reply

Please log in using one of these methods to post your comment:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s