Chapter 27: Structure and Bonding 1
Atomic Orbitals: Wave functions that represent the probability of finding electrons in a specific region of space s, p, d, f orbitals In organic chemistry, need to concentrate only on s and p orbitals 2
p orbitals have a nodal plane Area of space where the probability of finding electrons is almost zero (Figures 1.3 and 1.5) 3
Atomic orbitals can combine to generate new orbitals: ybridization: combination of atomic orbitals of the same atom producing new orbitals of lower energy sp, sp 2 and sp 3 These orbitals explain the geometry of molecules 4
Molecular orbitals are produced when atomic orbitals (either native or hybridized) of different atoms interact Produces bonding and anti-bonding orbitals (figure 1.10) 5
Molecular Orbitals: Sigma bonding (σ): ydrogen molecule Sigma bond: is covalent (most common bond in organic molecules) (Figure 1.13) 6
All single bonds are sigma bonds and all double or triple bonds contain only one sigma bond Sigma bonds can be formed from atomic orbitals, hybridized or native or a combination 7
2 other types of bonds in organic molecules: Pi (π) bond ydrogen-bond (-bond) 8
π-bond: cannot exists if a σ-bond is not already present Result from the overlap of p orbitals of two atoms. The π-bond is always perpendicular to the sigma bond connecting the nuclei Occur in sp 2 and sp hybridized atoms (double/triple bonds) 9
Double bond: C C ethylene 4 electrons in the bonding region between the nuclei first pair forms the sigma bond Second pair forms the pi bond Normal combination for a double bond such as the one found in ethylene 10
Triple Bond: 6 electrons in the bonding region between the nuclei first pair forms the sigma bond Second/third pairs form the pi bonds Normal combination for a triple bond such as the one found in acetylene C C acetylene 11
Molecular shapes and 3-D drawings Molecular shapes are related to hybridization of central atom sp 3 : only single bonds; 109.5 o angle (methane) sp 2 : double bond; 120 o angle (ethylene) sp: triple bond; 180 o angle (acetylene) 12
Only 4 general shapes are normally found in organic molecules: Linear (sp hybridized atoms) C C acetylene (easy to draw) 13
Trigonal planar (sp 2 hybridized) C C ethylene (easy to draw) 14
Trigonal pyramid or tetrahedral: (sp 3 hybridized) N sp3 hybridazation C ammonia methane harder to draw because of third dimension in plane going into the plane going out of the plane 15
Rotation of Single Bond vs Rigidity of Double Bond Consider ethane: C 3 -C 3 Each carbon is sp 3 Many structures of ethane are possible. They differ only by the position of one methyl group in relation to the other one: 16
CONFORMATIONS: Structures that differ only by rotation along a single bond Ex: ethane C C C C eclipsed form staggered form 17
Which of these 2 structures of ethane is the right one? 18
Both, and all other structures are correct. In fact a real ethane molecule rotates through all the conformations 19
Consider ethylene: C 2 =C 2 Ethylene is quite rigid and rotation along the C=C is not allowed. C C 20
In ethylene, rotation does not affect the sigma bond, but the pi overlap is broken RULE: Rotation is allowed along single bonds, but double/triple bonds are rigid and cannot be twisted 21
Because of this rigidity, compounds with different substituent arrangements on a double bond are different: 22
Example: 2-butene C 3 C=CC 3 C 4 8 C 3 C 3 C 3 C C C C C 3 cis-2-butene substituents (C 3 ) are on the same side of the C=C trans-2-butene substituents (C 3 ) are on opposite sides of the C=C 23
these 2 compounds are different and have different chemical/physical properties ISOMERS: Compounds having the same molecular formula, but different structures 24
Example: Butane C 4 10 C 3 C 3 C 2 C 2 C 3 C 3 CC 3 n-butane iso-butane 25
More accurately, these compounds are structural (constitutional) isomers, because they differ only in their bonding sequence. 26
Example 2: pentane C 5 12 : 3 structural isomers C 3 C 2 C 2 C 2 C 3 C 3 CC 2 C 3 C 3 C 3 C 3 CC 3 C 3 n-pentane isopentane neopentane 27
ow many structural isomers with a molecular formula C 6 14 can you draw? 28
Solution 29
Stereoisomerism Consider 2-butene: Are the 2 isomers of 2-butene structural isomers? C 3 C 3 C 3 C 3 cis trans 30
No, because they do not differ in their bonding sequence. The only difference is in the orientation of the C 3 groups attached to the double bond carbons. 31
This type of isomerism is called: STEREOISOMERISM And cis- and trans-2-butene are stereoisomers 32
Example: Identify the structural isomers and the stereosiomers among the compounds below. C 3 C 3 C 3 C 3 C 3 C 3 A B C 33
A/B and B/C are structural isomers A/C are stereoisomers Draw all the possible isomers of pentene (C 5 10 ) and identify the structural and stereoisomers. (Answer: stereo=2 structural=9) 34