Organic Chemistry Introduction and Lewis Structures

Valence Electrons and Bonding

• Group 1 Elements: Elements like hydrogen (H), lithium (Li), and sodium (Na) have one valence electron and typically form one bond.

  • Hydrogen (H) can only form one bond.

• Group 2 Elements: Beryllium (Be) has two valence electrons and typically forms two bonds.

• Group 3A Elements: Boron (B) tends to form three bonds when neutral.

  • If boron has a negative charge, it forms four bonds.

• Carbon: Carbon (C) has four valence electrons and ideally forms four bonds.

• Nitrogen: Nitrogen (N) has five valence electrons and typically forms three bonds in its neutral state to satisfy the octet rule.

• Oxygen: Oxygen (O) has six valence electrons and forms two bonds to achieve an octet.

• Halogens: Fluorine (F), chlorine (Cl), bromine (Br), and iodine (I) have seven valence electrons and usually form one bond.

  • Exceptions occur when these halogens are central atoms.

• Sulfur and Selenium: Elements under oxygen in the periodic table, like sulfur (S) and selenium (Se), typically form two bonds. Phosphorus (P) typically forms three.

Nitrogen and Oxygen Bonding Examples

• Ammonia (NH3): Nitrogen has one lone pair and three bonds, with no charge, representing its ideal state.

• If nitrogen loses a hydrogen, it will have two bonds and a negative charge.

• If nitrogen gains a hydrogen ion, it will acquire a positive charge and four bonds.

• Water (H2O): Oxygen forms two bonds with two hydrogen atoms and has two lone pairs; in this state, it carries no charge.

• Hydroxide (OH-): Hydroxide has a negative charge because oxygen only forms one bond.

• If oxygen forms three bonds, it will have a positive charge.

Quick Rules for Drawing Lewis Structures

• Nitrogen (N) typically forms three bonds and has one lone pair when neutral.

• Oxygen (O) usually forms two bonds and has two lone pairs.

• Fluorine (F) typically has one bond and three lone pairs.

• Carbon (C) always forms four bonds.

Alkane Lewis Structures

• Alkanes are organic molecules composed of carbon and hydrogen with single bonds only. They are saturated hydrocarbons.

• Example: Methane ($CH_4$ ) - Carbon forms four bonds, each with a hydrogen atom.

• Example: Ethane ($CH<em>3CH</em>3$) - Two carbon atoms, each with three hydrogen atoms attached, connected by a single bond.

• Example: Propane ($CH<em>3CH</em>2CH_3$) - A three-carbon chain where the terminal carbons each have three hydrogen atoms and the central carbon has two.

Alkene Lewis Structures

• Alkenes contain at least one carbon-carbon double bond and are unsaturated compounds.

• Example: $C<em>2H</em>4$ - Two carbon atoms, each with two hydrogen atoms, are connected by a double bond. Distribute hydrogen atoms equally among carbon to ensure stability.

Alkyne Lewis Structures

• Alkynes possess at least one carbon-carbon triple bond and are unsaturated hydrocarbons.

• Example: $C<em>2H</em>2$ - Two carbon atoms, each bonded to one hydrogen atom, are connected by a triple bond.

Bond Strength and Length

• Triple bonds are stronger but shorter than single bonds.

• Single bonds are longer but weaker.

• Analogy: Breaking one pencil (single bond) is easier than breaking three pencils (triple bond) simultaneously.

Common Alkane Names

• Methane ($CH_4$) - One carbon atom.

• Ethane (C2H6) - Two carbon atoms.

• General formula for alkanes: CnH{2n+2}

Alkene and Alkyne Formulas

• Ethene (C2H4), Alkene with two Carbon atoms.

• General formula for alkenes: CnH{2n}

• Ethyne (C2H2), Alkyne with two Carbon atoms

• General formula for alkynes: CnH{2n-2}

Naming Conventions for 3-10 Carbon Alkanes

• Propane ($C<em>3H</em>8$) - Three carbons. Followed by propene ($C<em>3H</em>6$) and propyne ($C<em>3H</em>4$) for alkene and alkyne

• Butane ($C<em>4H</em>{10}$) - Four carbons. Followed by butene and butyne for alkene and alkyne

• Pentane ($C<em>5H</em>{12}$) - Five carbons

• Hexane ($C<em>6H</em>{14}$) - Six carbons

• Heptane ($C<em>7H</em>{16}$) - Seven carbons

• Octane ($C<em>8H</em>{18}$) - Eight carbons

• Nonane ($C<em>9H</em>{20}$) - Nine carbons

• Decane ($C<em>{10}H</em>{22}$) - Ten carbons

Alcohols and Naming

• Alcohols contain an -OH (hydroxyl) group.

• Example: Methanol ($CH_3OH$)

  • A carbon atom attached to three hydrogen atoms bonded to an oxygen with a hydrogen atom.

  • The "meth" prefix indicates one carbon atom, and "ol" signifies an alcohol.

Examples and Naming

• Example: $CH<em>3CH(OH)CH</em>3$

  • A carbon with three hydrogen atoms, bonded to a carbon with one hydrogen and an -OH group, and then to another carbon with three hydrogen atoms.

  • IUPAC name: 2-propanol because the -OH group is on the second carbon.

Ethers

• Ethers contain an oxygen atom bonded to two alkyl groups (R-O-R).

• Example: $CH<em>3OCH</em>3$ (Dimethyl Ether)

  • Structure: A central oxygen atom connected to two methyl groups ($CH_3$).

• Example: $CH<em>3CH</em>2OCH_3$ (Ethyl Methyl Ether)

IUPAC Naming of Ethers

• Example: Ethyl Methyl Ether can also be named as 1-methoxyethane.

  • The longest carbon chain is ethane, and the methoxy group ($OCH_3$) is the substituent.

Ethoxy Propane

• Common Name: Ethyl Propyl Ether

• IUPAC Name: 1-ethoxypropane

  • An ethoxy group ($OCH<em>2CH</em>3$) attached to a three-carbon chain (propane).

Ketones

• Ketones feature a carbonyl group (C=O) in the middle of the carbon chain.

• The carbonyl group connects carbon to oxygen via a double bond with two lone pairs on oxygen.

• Example: Butanone

Naming Ketones

• To name, identify the longest chain containing the carbonyl group and replace the -e ending of the alkane name with -one.

• Indicate the position of the carbonyl group with a number if necessary.

• Example: 3-Heptanone - A seven-carbon chain with the carbonyl group on the third carbon atom.

Aldehydes

• Aldehydes have a carbonyl group (C=O) at the end of the carbon chain. Represented by CHO group.

• Example: Ethanal ($CH_3CHO$) - A two-carbon aldehyde.

• To name, replace the -e ending of the corresponding alkane name with -al

• The aldehyde group is always on carbon one, so no need to specify it in the name.

• Example: Pentanal - A five-carbon chain with an aldehyde group at the end.

Carboxylic Acids

• Carboxylic acids contain a carboxyl group (COOH), which is a carbonyl group and a hydroxyl group (OH) bonded to the same carbon atom.

• General formula: R-COOH (or R-CO2H).

• Example: Propanoic acid ($CH<em>3CH</em>2COOH$) - A three-carbon carboxylic acid.

• To name, replace the -e ending of the corresponding alkane name with -oic acid.

• Example: Octanoic acid - An eight-carbon carboxylic acid.

Ester

• Esters have the functional group R-COOR', where a carbon is double-bonded to an oxygen and single-bonded to another oxygen that is connected to an alkyl group. Carboxylic acid where hydroxyl hydrogen is subbed with an alkyl group.

• Example: Methyl ethanoate ($CH<em>3CO</em>2CH_3$)

  • The part of molecule that has only one oxygen is called methyl group.

  • The side with the carbonyl group consists of ethanoate which is associated with two carbons, including the one that has two oxygens.

• The alkyl group attached to the single-bonded oxygen is named first, followed by the name of the carboxylic acid derivative.

Amines

• Amines contain a nitrogen atom bonded to one, two, or three alkyl groups.

• General formula: R-NH2.

• Example: Ethylamine ($CH<em>3CH</em>2NH_2$)

  • IUPAC name: Aminoethane.

• Example: 2-aminopentane - An amino group ($NH_2$) on the second carbon of a five-carbon chain.

Amides

• Amides feature a carbonyl group bonded to a nitrogen atom (R-CO-NH2).

• Example: Butanamide. Which contains four carbons.

  • To name, replace -ane of corresponding alkane with amide

Additional Functional Groups

• Nitriles (R-CN).

• Acid chlorides (R-COCl).

• Benzene ring ($C<em>6H</em>6$).

  • Also known as an aromatic ring

  • A six-carbon ring with alternating single and double bonds. Every carbon atom has one hydrogen atom attached to it.

Formal Charge Calculation

• Formal Charge = (Number of valence electrons in free atom) - (Number of bonds + Number of lone pair electrons)

Oxygen Formal Charge

• Example: Oxygen with One Bond and Three Lone Pairs

  • Formal Charge = 6 - (1 + 6) = -1

• Example: Oxygen with Three Bonds and One Lone Pair

  • Formal Charge = 6 - (3 + 2) = +1

Nitrogen Formal Charge

• Example: Nitrogen with Two Bonds and Two Lone Pairs

  • Formal Charge = 5 - (2 + 4) = -1

Resonance Structures

• Resonance structures are different ways of drawing the same molecule by moving electrons, not atoms.

• Curve arrow notation is used to represent the movement of electrons: a full arrow represents two electrons, and a half arrow represents one electron.

Acetate Example

• Acetate (ethanoate) ion: A negative charge on one oxygen can be delocalized by moving a lone pair to form a double bond and moving the double bond electrons to the other oxygen.

Amide Example

• Amide: A lone pair from nitrogen can form a pi bond with the carbonyl carbon, and the carbonyl pi bond breaks, placing a negative charge on the oxygen.

Major and Minor Resonance Contributors

• Major resonance contributors are more stable, while minor resonance contributors are less stable.

• Separation of charge in a resonance structure reduces stability, making it a minor contributor.
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Carbanion Example

• Carbanion structure: A lone pair adjacent to a double bond can be moved to create a new double bond, pushing electrons from the original double bond onto an adjacent carbon, resulting in a new carbanion.

Carbocation Resonance

• Carbocation: A positive charge on a carbon can be delocalized by moving a pi bond towards it, creating a new carbocation on a different carbon.

Alkane IUPAC Nomenclature

Steps

1. Identify Parent Chain: Find the longest continuous carbon chain.

2. Number the Parent Chain: Give the substituents the lowest possible numbers.

3. Name Substituents: Identify and name the groups attached to the parent chain.

4. Arrange Substituents: List substituents in alphabetical order with appropriate position numbers.

• Example: 3-methylhexane has a six-carbon chain (hexane) with a methyl group on the third carbon.

Example:

• 3,4-dimethylheptane has a seven-carbon chain with methyl groups on the third and fourth carbon atom

• Use commas to separate numbers and hyphens to separate numbers from letters.

More IUPAC Naming Conventions

• Example: 5-ethyl-4-methyloctane This contains an eight carbon chain, an ethyl group, and a methyl group. Ethyl is placed before methyl because it comes first alphabetically.

  • It's always preferable to put the numbers in ascending order.

  • The lowest number is always preferred.