UMass Amherst CHEM 261 - Organic Chemistry Annotations 261 (4 pages)

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Organic Chemistry Annotations 261



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Organic Chemistry Annotations 261

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School:
University of Massachusetts Amherst
Course:
Chem 261 - Organic Chemistry
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Basic Hybridization Lewis Structures Sp1 or Sp have 2 total things attached atoms lone pairs Sp2 have 3 total things attached atoms lone pairs Sp3 have 4 total things attached atoms lone pairs Orbital diagrams To draw an orbital diagram find the hybridization of each atom Then draw the valence orbitals for each atom Valence orbitals Sp1 has 2 valence Sp orbitals Sp2 has 3 valence Sp2 orbitals Sp3 has 4 valence Sp3 orbitals Then determine which orbitals overlap to form bonds Sigma bonds happen with an s orbital overlaps a p orbital Pi bonds happen when a p orbital overlaps lengthwise side by side with another p orbital Single bonds are Always sigma bonds A double bond has one sigma and one pi bond A triple bond has one sigma and two pi bonds There is not automatically an s orbital on the middle atom The s orbitals will hang out at the end of each p orbital in a small overlap This is the orbital diagram for methane CH4 it is an Sp3 hybrid orbital it has 4 sigma bonds and no pi bonds Formal charge is easy it s just Formal charge of an atom number of valence electrons dots sticks Carbon on the periodic table is a valence number 4 Neutral Carbon has a neutral formal charge of 0 a cationic charge of 1 and an anionic charge of 1 Ionic 3 charge Nitrogen is a valence number 5 Neutral nitrogen has a formal charge of 0 a cationic formal charge of 1 and an anionic formal charge of 1 Ionic 3 charge Oxygen is valence number 6 a neutral formal charge of 0 a cationic formal charge of 1 and an anionic charge of 1 To find out how many lone pairs an atom has just rearrange the formula so that it says dots valence electrons formal charge sticks Resonance To Draw Resonance Structures memorize these rules Don t move single bonds Lewis structures describe single bonds correctly You only move lone pairs or double or triple bonds Make sure all resonance structures are valid Lewis structures Don t break the octet rule f or any of the resonance structures lone pairs sticks can t be above four Don t move atoms The only thing that changes is the distribution of pi orbitals double bonds triple bonds and lone pair electrons can change Remember charges Sum of the formal charges for each molecule should stay the same you can t lose any electrons in going from one resonance structure to the next Of course the total number of charges may change from one resonance structure to another The electronegative atom should become more negative If the electronegative atom started positive it should become neutral in the resonance structure A resonance structure that has no charges contributes more to the overall hybrid than a resonance structure that has two Negative charges prefer electronegative atoms Positive charges prefer electropositive atoms Complete octets are more stable than incomplete octets Chirality Chiral molecules usually contain at least one carbon atom mixed with four non carbon non matching other atoms The carbon atom is called a stereo genic or c hiral center Any molecule that has at least one chiral center will be chiral Chiral molecules are not superimposable on their mirror image Atoms that are not chiral centers have two or more identical groups All methyl groups CH 3 groups methylene groups CH 2 groups or carbons involved in a double or triple bond can t be chiral centers because these atoms can t have four different groups attached to them Meso Meso compounds are different do not have one chiral center Locating Chirality Centers Optical Activity Calculation of Optical Rotation Cahn Ingold Prelog Sequence Rules Prioritize the groups from 1 to 4 based on the atomic number of the first atom attached to the chiral center The highest priority 1 goes to the group whose first atom has the highest atomic number the lowest priority 4 goes to the group whose first atom has the lowest atomic number Ties can break in two ways You reach a higher priority atom on one of the chains giving that group the higher priority A carbon attached to an oxygen has priority over a carbon attached to another carbon because oxygen has a higher atomic number than carbon Oxygen is atomic number 8 and carbon is atomic number 4 Atomic number is number of protons There are a larger number of identical atoms attached to one group over another Thus a carbon attached to two other carbons has a higher priority than a carbon attached to just one other carbon However atom type trumps the number of groups so a carbon attached to an oxygen has higher priority than a carbon attached to two or three other carbons Because oxygen has a higher atomic protein number Chirality Determining Whether Structures are Enantiomers or Identical molecules with different stereochemistry are called stereoisomers Specifying Absolute Configuration in JMOL Images Determining R S Configurations the labels R and S which stand for rectus right and sinister left Drawing Chiral Molecules Stereochemistry Chiral Achiral or Meso Structure different stereochemistry or differing spatial arrangements of atoms


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