1 Chapter Outline 4.1 Formal Charge Structural Formulas 4.2 Polar Covalent Bonds, Shape, and Polarity 4.3 Noncovalent Interactions 4.4 Alkanes 4.5 Constitutional Isomers 4.6 Conformations 4.7 Cycloalkanes 4.8 Alkenes, Alkynes, and Aromatic Hydrocarbons

2 Draw Lewis structure of molecules and calc. Formal Charge of an atom in the molecule: CH 4 NCl 3 HCN NH 4 + NO 3 - Formal charge (FC) – the charge of an atom in a molecule taken that all its bonded electrons are shared evenly between other atoms that it is bonded to. FC = (# of val. e-) – [ (# of atom’s e- around the atom itself ] Or FC = (# of val. e-) – [ (# of lone pair e-) + ½ ( # of bonded e-) ] What does FC help me about a molecule?

3 How do we draw a structural formula? For small molecules, knowing the number of covalent bonds that an atom is expected to form can be a good place to start. A carbon atom can form 4 covalent bonds. A nitrogen atom can form 3 covalent bonds. An oxygen atom can form 2 covalent bonds. A halogen atom can form 1 covalent bond. A hydrogen atom can form 1 covalent bond.

4 Let’s Try It! The line-bond structure of acetate ion is shown here. Assign formal charges to the atoms in this ion.

5 4.2: Structural Formula of organic compounds Polar covalent bonds The structure of an organic molecule can be represented by an electron dot structure (Lewis Structure) or a line-bond structure (structural formula). Structural formula Lewis structure of the molecule Molecular formula is C 3 H 8 O.

6 Alkanes are molecules that consist only of carbon and hydrogen atoms and contain only single bonds. Examples include: methane (CH 4 ) ethane (CH 3 CH 3 ) propane (CH 3 CH 2 CH 3 ) butane, pentane, hexane, heptane, octane, nonane, decane These are examples of normal alkanes, which means that their carbon chains are unbranched. 4.4: Alkanes (alkanes are hydrocarbons, contain only carbon and hydrogen atoms.)

7 sp 3 carbon; Bonding for C with four single bonds: What is the molecular shape or geometry of the sp 3 hybrid orbital?

8 Figure 4.10 Alkanes

9 General Properties of Alkanes Alkanes contain only nonpolar covalent bonds, are nonpolar molecules, and are attracted to one another by London forces. The more carbon atoms in a normal alkane, the higher its boiling point. The longer the alkane, the greater it’s surface area and the stronger the London forces that hold it to other molecules. They are not very reactive, except for combustion reaction.

10 4.3: Types of Noncovalent interactions: Noncovalent interactions are interactions that do not involve the sharing of valence electrons (or direct bonding). There are those noncovalent interactions due to the attraction of permanent charges. Hydrogen bonds Salt bridges Dipole-dipole interactions Ion-dipole interactions London forces

11 Figure 4.8 (a) hydrogen bonds (b) salt bridges (c) dipole-dipole forces (d) ion-dipole interactions (e) coordinate covalent bonds

12 Figure 4.9

13 IUPAC Rules for Naming Alkanes 1. Identify and name the parent or main chain. 2. Identify and name any alkyl groups attached to the main chain. 3. Determine the point of attachment of alkyl groups to the main chain. 4. Construct the name of the alkane by placing the alkyl groups in alphabetical order and specify their position numbers, followed by the names of the main chain. The labels di, tri, tetra, etc., are added if two or more identical substituents are present. 5.The lowest number is assigned to the multiply-bonded carbon atoms if there are two competing branches.

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15 Example: Give the correct IUPAC name for this molecule.

16 Let’s Talk About It! Write the correct IUPAC name for each molecule.

17 Constitutional isomers of C 4 H 10. Note: Constitutional isomers have the same formula but different names. Molecules that have the same molecular formula, but different atomic connections (or bonding) are called constitutional isomers. 4.5: Isomers:

18 Example 1: Let’s Try It! Are the two alkanes constitutional isomers or are they identical? Example 2 :

19 Rotation about single bonds allows most molecules to assume a number of different 3-dimensional shapes. The shapes that a molecule can be in due to bond rotations are called conformations. The different conformations of a molecule –have the same molecular formula –have the same atomic connections –have a different 3-dimensional shape –are interchanged by the rotation of single bonds 4.6: Conformations

20 Figure 4.13 Conformations of Butane Rotation about the bond between carbons 2 and 3 in butane gives rise to different conformations for the molecule. Why should I care about conformation?

21 In some alkanes the carbon atoms are joined into rings. These are called cycloalkanes. 4.7: Cycloalkanes

22 When naming cycloalkanes, the ring is usually designated as the parent chain or main chain, which is named by combining “cyclo” with the appropriate prefix name. When only one branch is attached to a cycloalkane, the carbon atom of the ring that is bondedd to the branch is assigned carbon #1, but we don’t write it. Why? When a ring holds more than one branches, the ring is numbered from the position and in the direction that gives the lowest numbers to the branches. If one of the branches’s name starts with “e” and a second one with “m”, assign #1 to the one that starts with “e”, so as to alphabetize the first letter of the branch names, but still count in the direction that will result in the lowest numbers.

23 Stereoisomers The limited rotation of the carbon-carbon single bonds in cycloalkanes has an interesting side effect in that it allows for the existence of stereoisomers, molecules that –have the same molecular formula –have the same sequence of atomic connections –have a different 3-dimensional shape –are interchanged only by breaking bonds

24 Geometric Isomers When stereoisomers exist because of restricted bond rotation, the stereoisomers are called geometric isomers. Geometric isomers come in pairs – one is called cis and another trans.

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26 Unsaturated Hydrocarbons Alkenes contain carbon-carbon double bonds. Alkynes contain carbon-carbon triple bonds. Aromatic compounds contain benzene rings. 4.8: Alkenes, Alkynes, and Aromatic Compounds

27 Alkenes contain C=C Alkynes contain C≡C

28 sp 2 carbon (Bonding in the C=C) What geometrical shape is the sp 2 carbon?

29 sp carbon (Bonding in the triple bond): What geometrical shape exists around the C with a triple bond?

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31 Properties of Unsaturated Hydrocarbons Like alkanes, the unsaturated hydrocarbons have only nonpolar covalent bonds and are nonpolar molecules. London forces hold members of these hydrocarbon families to one another. Increasing size leads to stronger London force attractions between molecules and higher melting and boiling points. They undergo combustion reaction well like the alkanes, and plus they are more reactive.

32 Naming alkenes and alkynes 1. When using IUPAC rules to name alkenes or alkynes, the main chain is the longest chain of carbon atoms that contains the carbon-carbon double or triple bond. 2. Begin numbering the parent chain at the end nearer to the double or triple bond. Alkenes end with “ene” Alkynes end with “yne”

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34 Aromatic compounds:

35 Figure 4.20 Polycyclic Aromatic Hydrocarbons (PAHs)