Section 1: Organic Structure and Bonding

Transcription

1 Section 1: Organic Structure and Bonding What is Organic Chemistry? Compounds containing only carbon and hydrogen, also known as, are the simplest form of organic compounds. Examples: C C C C C C Atoms other than C and are referred to as. Common heteroatoms encountered in organic chemistry:. Example of an Organic Molecule containing a heteroatom: Drawing Organic Molecules using Ethanol as an example: (a) Lewis-Dot Structure (b) Molecular Formula (c) Condensed Structural Formula (d) Line-Angle formula (e) Abbreviated 1

2 Reviewing Lewis Dot Structures In Lewis Dot Structures, the valence electrons are shown as dots surrounding the atom. Example: In ionic bonds the electrons are transferred between atoms. In covalent bonds the electrons are shared between atoms and are shown in the Lewis structures as a line between atoms. Most atoms follow the octet rule which states that an atom is in the lowest energy state when it has eight valence electrons. Question: Examine the Lewis structures of the following molecules: NO 3 NO 2 2 SO 4 2 CO 3 C 3 CO 2 C 3 (C 3 ) 2 CO C 3 CO C 3 SC 3 Which of the Lewis structures above violate the octet rule? 2

3 Formal Charge Formal charge = group number (from periodic table) # of lines # of dots Question: Identify the formal charge for each of the following central atoms: C N O Br C N O Br C N O Br Which ones violate the octet rule? What bonding trends can we establish based on formal charge? Isomers Example: Ethanol and dimethyl ether are isomers with the molecular formula C 2 6 O. Draw the structural formula of each isomer. 3

4 Resonance structures are different ways of drawing the Lewis structure of the same molecule. The are all in the same place, but the have moved. Example: Draw another resonance structure for each of the following compounds: Common Resonance Pattern: Common Resonance Pattern: Common Resonance Pattern: Determining the Best Resonance Contributors A resonance contributor is more important when: Identify the more important resonance contributor in each of the cases above. 4

5 The Resonance ybrid Rules for Drawing the Resonance ybrid 1) Copy everything that is identical in all resonance contributors. 2) Add dotted bonds for any bonds that are present in only some resonance contributors 3) Add partial positive (δ+) or partial negative (δ ) charges for any charges that are present in only some resonance contributors 4) Omit any lone pairs that are present in only some resonance contributors Example: Draw the resonance hybrid for each compound shown in the previous problem. 5

6 Simplifying Lewis Structures Organic compounds have so many carbons and hydrogens that it can be helpful to make them assumed in order to clean up the drawing. Compare: Lewis Structure Condensed Structure Skeletal Structure Drawing / Interpreting Skeletal Structures The end of each line or a vertex represents a carbon atom that has four bonds. These carbon atoms do not need to be shown. If (and only if) the carbon atom is not shown, then the hydrogen atoms connected to that carbon atom do not need to be shown. All heteroatoms (any atom that is neither carbon nor hydrogen) must be shown and all hydrogens connected to heteroatoms must be shown. All formal charges must be shown, but lone pairs of electrons are optional. Question: Convert the following condensed structures into skeletal structures: (C 3 ) 3 CBr C 4 10 (straight chain) C 6 6 (benzene) C 3 CO Question: Convert the following skeletal structures into Lewis structures: 6

7 Electronegativity Trend: Note: The electronegativity values on the Pauling Scale are included, above. Pneumonic for Organic Chemistry: Question: Use the partial charges (δ+ and δ ) to indicate the dipole moment in F. VSEPR Structures The electron geometry is determined by the number of groups of electrons (lone pairs of electrons or bonds) around the central atom. Linear Trigonal Planar Tetrahedral The molecular geometry is determined by the position of the atoms (ignore the lone pairs). In addition to the above, we have these possible molecular geometries: 7

8 Bent Trigonal Pyramidal Bond Angle Distortion The bond angles shown above are only correct when all electron groups attached to a central atom are identically sized. Otherwise, larger groups will occupy more space and distort the bond angles. Sizes of Electron Groups: Groups of atoms > Lone Pairs of Electrons > Single Atoms Recall: Single atoms get larger from top to bottom and right to left in periodic table!!! Representing Three Dimensional Molecules on Paper We use the following symbols to represent bonds in molecules: Straight Line Wedge Dash Bond in Plane Bond Extends Bond Extends of Paper Out of Paper Into Paper (From L to R) (From L to R) Linear Bent Trigonal Tetrahedral Trigonal Planar Pyramidal Question: Draw the molecular structure for each of the following molecules, using dashes and wedges to indicate the three dimensional nature of each molecule and label the bond angles: C 4 N 3 Acetic Acid (C 3 CO 2 ) 8

9 Polar Molecules A molecule is polar (has a net dipole moment) if all of the dipole moments do NOT cancel out in three dimensions. Otherwise, the molecule is nonpolar Question: Revisit the Lewis structure of methane and ammonia. Which one is polar? ybrid Orbitals Recall: A covalent bond is formed by overlapping 2 orbitals one from each atom. Most of the atoms we encounter in organic chemistry are period 2 elements and therefore have s and p orbitals. Atomic Orbitals ybridization and Geometry Shapes sp 3 = tetrahedral remaining p orbitals sp 2 = planar remaining p orbitals s p sp sp 2 sp 3 sp = linear remaining p orbitals ydrogen only has an orbital to use to make covalent bonds. Example: Label the atomic orbitals on each atom. C 3 3 C C C C O C 3 9

10 Molecular Orbitals: sigma pi The hybrid orbitals overlap to form bonds The leftover p orbitals overlap to form bonds Exercise List the atomic orbitals used in the following examples: C C C BOND NOTATION: (Atom orb Atom orb ) Ex: (C sp 2 s ) Exercise: each C. Describe atomic orbitals, Molecular orbitals, and geometry around C 3 C C Summary If an atom has 4 electron groups, the hybridization is and it can form π bonds If an atom has 3 electron groups, the hybridization is and it can form π bonds If an atom has 2 electron groups, the hybridization is and it can form π bonds 10

11 Question: Assign the hybridization of every atom in each of the following: Question: Consider this molecule, 2 C=C=C 2 (allene). What is the C C C bond angle? What is the C bond angle? Why is this molecule NOT flat? Sketch a picture: 11

12 Bond Lengths and Bond Strengths For each set of bonds, indicate which are longer and which are stronger: a) C C C=C C C b) C F C Br C I c) C sp 3 C sp 2 C sp d) C C C N C O Trends: Reading: Wade Chapters 1 & 2 omework: Chapter 1-21,22,23,25 31, 34 37, 40, 41 Chapter 2-28, 29, 30, 31 12