Chapter Outline

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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

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Draw Lewis structure of molecules and calc. Formal Charge of an atom in the molecule:

CH4 NCl3 HCN NH4+ NO3

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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?

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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.

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Let’s Try It!

The line-bond structure of acetate ion is shown here. Assign formal charges to the atoms in this ion.

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4.2: Structural Formula of organic compounds4.2: Structural Formula of organic compoundsPolar covalent bondsPolar 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 formulaLewis structure of the molecule

Molecular formula is C3H8O.

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• Alkanes are molecules that consist only of carbon and hydrogen atoms and contain only single bonds.

• Examples include:

methane (CH4)

ethane (CH3CH3)

propane (CH3CH2CH3)

butane, pentane, hexane, heptane, octane, nonane, decane• These are examples of normal alkanes, which means that

their carbon chains are unbranched.

4.4: Alkanes4.4: Alkanes(alkanes are hydrocarbons, contain only carbon and hydrogen (alkanes are hydrocarbons, contain only carbon and hydrogen atoms.)atoms.)

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sp3 carbon; Bonding for C with four single bonds:

What is the molecular shape or geometry of the sp3 hybrid orbital?

8Figure 4.10 Alkanes

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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.

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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

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

12Figure 4.9

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IUPAC Rules for Naming Alkanes1. 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.

CH3CHCH2CH2CH2CH3

CH3

5. The lowest number is assigned to the multiply-bonded carbon atoms if there are two competing branches.

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Example:

Give the correct IUPAC name for this molecule.

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Let’s Talk About It!

Write the correct IUPAC name for each molecule.

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Constitutional isomers of C4H10.

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:

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Example 1: Let’s Try It!

Are the two alkanes constitutional isomers or are they identical?

Example 2:

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• 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: Conformations4.6: Conformations

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Figure 4.13 Conformations of ButaneRotation about the bond between carbons 2 and 3 in butane gives rise to different conformations for the molecule.

Why should I care about conformation?

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• In some alkanes the carbon atoms are joined into rings. These are called cycloalkanes.

4.7: Cycloalkanes4.7: Cycloalkanes

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• 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.

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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

Br C

H

ClF

C

H

ClF

Br

CH

Cl

F

Br C H

Cl

F

Br

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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.

C CCH3

HH

CH3

C CCH3H

HCH3

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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 Compounds4.8: Alkenes, Alkynes, and Aromatic Compounds

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Alkenes contain C=C Alkynes contain C≡C

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sp2 carbon (Bonding in the C=C)

What geometrical shape is the sp2 carbon?

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sp carbon (Bonding in the triple bond):

What geometrical shape exists around the C with a triple bond?

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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.

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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”

H2C C CH2CH3 CH3CHCH2CH

CH3

CH2 CH2 CH CHCH2CH2CH3

CH2CH2CH3

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CH3 CH3

CH3

CH3

CH3

CH3

CH3

CH2OH C OH

O

C O

O

CH2CH3

Aromatic compounds:

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Figure 4.20 Polycyclic Aromatic Hydrocarbons (PAHs)