Organic molecules Contain c and h, as well aso,n.s. etc Carbon chain as the main framework Alkyl (only c and H elements) Alkane(single C-C bond only) Alene(contains at least one C=C bond) Alkyne(contains at least one C=C bond) ■ Examples: Write Lewis structures: CHa(methane), C2H(ethane), C3H (propane), C2Ha(ethylene), C2H2(ethyne), C2H5OH (ethanol) CH3COCH3(acetone)
Organic Molecules Contain C and H, as well as O, N, S, etc. Carbon chain as the main framework Alkyl (only C and H elements) Alkane (single C-C bond only) Alene (contains at least one C=C bond) Alkyne (contains at least one CΞC bond) Examples: Write Lewis structures: CH4 (methane), C2H6 (ethane), C3H8 (propane), C2H4 (ethylene), C2H2 (ethyne), C2H5OH (ethanol), CH3COCH3 (acetone). 11
Exception of Octet Rule in Lewis structure More than 8 electrons in one of the atoms EXample: SF6, PCIs Fewer than 8 electrons in one of the atoms Example: BeCl2, BF3 Odd number of electrons in one of the atoms EXample: NO, NO
Exception of Octet Rule in Lewis Structure More than 8 electrons in one of the atoms; Example: SF6, PCl5. Fewer than 8 electrons in one of the atoms; Example: BeCl2, BF3. Odd number of electrons in one of the atoms Example: NO, NO2. 12
Formal Charges Formal charges(FC): showing approximate distribution of electrons in molecules and ions Calculate fc Assign each atom half of e in shared e-pairs Assign all unshared e as where they are Subtract the number of electrons assigned to each atom from the number of its valence electrons Example CO,CO,NO2,HN○3
Formal Charges Formal charges (FC): showing approximate distribution of electrons in molecules and ions. Calculate FC: Assign each atom half of e in shared e-pairs; Assign all unshared e as where they are; Subtract the number of electrons assigned to each atom from the number of its valence electrons Example: CO, ClO- , NO2, HNO3. 13
FC for deciding the best lewis structure Rules Book, pg 324 Small FC are better than large FC. Zero FC is the best. Negative FC on atom(s)with higher electronegativity Opposite charges closer together are more likely than separating far apart; Same charges in adjacent atoms are very unlikely ● EXamples: (1)Decide which Lewis Structure of co, is better? (2) Decide formaldehyde(CH2O) structure(Book pg 324: Q 9.9)
FC for Deciding the Best Lewis Structure Rules (Book, pg 324) Small FC are better than large FC. (Zero FC is the best.) Negative FC on atom(s) with higher electronegativity. Opposite charges closer together are more likely than separating far apart; Same charges in adjacent atoms are very unlikely. Examples: (1) Decide which Lewis Structure of CO2 is better? (2) Decide formaldehyde (CH2O) structure (Book pg. 324: Q 9.9). 14
Resonance Structure Resonance structures Possible electron structures of multiple identical Lewis structures Arbitrary representation, not real ■ Examples:
Resonance Structure Resonance structures Possible electron structures of multiple identical Lewis structures Arbitrary representation, not real Examples: 15