Organic Chemistry: Hydrocarbons and Functional Groups

Condensed Structural Formulas and Line-Angle Structures

Organic molecules can be represented in several ways. Condensed structural formulas show all atoms in a molecule, often omitting some or all of the bonds. Line-angle structures (skeletal formulas) use lines to represent carbon-carbon bonds, with vertices and line ends representing carbon atoms. Hydrogen atoms attached to carbons are usually omitted for simplicity.

• Condensed formula example: CH3CH2OH (ethanol)

• Line-angle structure: Each vertex or line end is a carbon; hydrogens are implied.

Types of Hydrocarbons

Hydrocarbons are organic compounds composed only of carbon and hydrogen. They are generally nonpolar due to similar electronegativities of C and H.

• Alkanes: Saturated hydrocarbons with only single bonds. General formula:

• Cycloalkanes: Saturated hydrocarbons with carbon atoms arranged in a ring. General formula:

• Alkenes: Unsaturated hydrocarbons with at least one carbon-carbon double bond. General formula:

• Alkynes: Unsaturated hydrocarbons with at least one carbon-carbon triple bond. General formula:

• Benzene ring: A six-carbon ring with alternating double bonds (aromatic compound).

Saturated hydrocarbons contain only single bonds (alkanes, cycloalkanes). Unsaturated hydrocarbons contain double or triple bonds (alkenes, alkynes, aromatics).

IUPAC Nomenclature of Alkanes and Cycloalkanes

The IUPAC system provides rules for naming organic compounds:

• Identify the longest continuous carbon chain (parent chain).

• Number the chain to give substituents the lowest possible numbers.

• Name and number substituents (alkyl groups, halogens, etc.).

• For cycloalkanes, the ring is the parent unless a longer chain is attached.

Example: 2-methylpropane (isobutane): three-carbon chain with a methyl group on the second carbon.

Isomerism

Isomers are compounds with the same molecular formula but different structures.

• Structural (constitutional) isomers: Differ in the connectivity of atoms.

• Conformational isomers (conformers): Differ by rotation around single bonds.

• Stereoisomers: Same connectivity, different spatial arrangement.

  • Cis-trans (geometric) isomers: Occur in cycloalkanes and alkenes due to restricted rotation.

  • Enantiomers: Non-superimposable mirror images; require a chiral center (carbon with four different groups).

  • Diastereomers: Stereoisomers that are not mirror images (see Chapter 6).

Functional Groups

Recognize the following functional groups in structures:

• Alkane: Only single C–C and C–H bonds

• Alkene: C=C double bond

• Alkyne: C≡C triple bond

• Alcohol: –OH group

• Ether: R–O–R'

• Aldehyde: –CHO group

• Ketone: C=O group within carbon chain

• Carboxylic acid: –COOH group

• Ester: –COOR group

• Amine: –NH2, –NHR, or –NR2

• Amide: –CONH2, –CONHR, or –CONR2

Thermochemistry and Chemical Reactions

Exothermic vs. Endothermic Reactions

Exothermic reactions release heat to the surroundings (). Endothermic reactions absorb heat from the surroundings ().

• Example (exothermic): Combustion of methane:

• Example (endothermic): Photosynthesis:

Spontaneous vs. Nonspontaneous Processes

Spontaneous (exergonic) processes occur without outside intervention (). Nonspontaneous (endergonic) processes require energy input ().

• Free energy (): Determines spontaneity of a process.

• Activation energy: Minimum energy required to initiate a reaction.

Caloric Values in Food

Different macronutrients provide different amounts of energy:

• Carbohydrates: 4 Cal/g

• Proteins: 4 Cal/g

• Fats: 9 Cal/g

Example: 10 g of fat provides Calories.

Kinetics: Factors Affecting Reaction Rate

• Temperature: Higher temperature increases reaction rate.

• Concentration: Higher reactant concentration increases rate.

• Catalyst: Lowers activation energy, increasing rate without being consumed.

Types of Chemical Reactions

• Synthesis (combination): Two or more substances combine to form one product.

• Decomposition: One substance breaks down into two or more products.

• Single exchange (displacement): One element replaces another in a compound.

• Double exchange (displacement): Two compounds exchange ions.

Combustion Reactions

Combustion involves a hydrocarbon reacting with O2 to produce CO2 and H2O. Always balance the equation.

• Example: (then multiply by 2 to avoid fractions)

Reversible vs. Irreversible Reactions

• Reversible: Can proceed in both directions; indicated by a double arrow ().

• Irreversible: Proceeds only in one direction; indicated by a single arrow ().

Chemical Equilibrium

Chemical equilibrium is the state where the rates of the forward and reverse reactions are equal, and concentrations of reactants and products remain constant.

Oxidation-Reduction (Redox) Reactions

• Oxidation: Loss of electrons (increase in oxidation number).

• Reduction: Gain of electrons (decrease in oxidation number).

• Oxidizing agent: Causes oxidation, is reduced.

• Reducing agent: Causes reduction, is oxidized.

• Inorganic redox: Based on electron transfer.

• Organic redox: Often involves gain/loss of oxygen or hydrogen.

Organic Reaction Types

• Condensation: Carboxylic acid + alcohol → ester + water.

• Hydrolysis: Ester + water → carboxylic acid + alcohol.

• Hydrogenation: Addition of H2 to a double bond (alkene → alkane).

• Hydration: Addition of H2O to a double bond (alkene → alcohol). Markovnikov’s Rule: In addition of HX or H2O to an alkene, the hydrogen attaches to the carbon with more hydrogens already attached.

Carbohydrates: Structure and Classification

Classification by Sugar Units

• Monosaccharide: 1 sugar unit (e.g., glucose)

• Disaccharide: 2 sugar units (e.g., sucrose)

• Oligosaccharide: 3–10 sugar units

• Polysaccharide: >10 sugar units (e.g., starch, cellulose)

Alcohol Classification

• Primary (1°) alcohol: –OH group attached to a carbon bonded to one other carbon.

• Secondary (2°) alcohol: –OH group attached to a carbon bonded to two other carbons.

• Tertiary (3°) alcohol: –OH group attached to a carbon bonded to three other carbons.

Fischer Projections of Common Monosaccharides

• D-glucose, D-galactose, D-mannose, D-fructose: Know their Fischer projections (linear representations showing stereochemistry).

Functional Group and Carbon Number in Sugars

• Aldose: Monosaccharide with an aldehyde group.

• Ketose: Monosaccharide with a ketone group.

• Classification example: "Aldohexose" = 6-carbon sugar with an aldehyde group.

Enantiomers: D- and L-Sugars

• Enantiomers: Non-superimposable mirror images.

• D-sugar: –OH on the highest-numbered chiral carbon is on the right in Fischer projection.

• L-sugar: –OH on the left.

Epimers and Anomers

• Epimer: Stereoisomers differing at only one chiral center (not the anomeric carbon).

• Anomer: Isomers differing at the anomeric carbon (carbonyl carbon in ring form).

• Alpha (α) anomer: –OH on anomeric carbon is trans to CH2OH group.

• Beta (β) anomer: –OH on anomeric carbon is cis to CH2OH group.

Glycosidic Bonds

Glycosidic bonds link monosaccharides in disaccharides and polysaccharides. Named by the carbons involved (e.g., α(1→4)).

Disaccharides and Polysaccharides

Reducing vs. Non-Reducing Sugars

• Reducing sugars: Contain a free anomeric carbon (e.g., glucose, maltose, lactose).

• Non-reducing sugars: No free anomeric carbon (e.g., sucrose).

Blood Types and Carbohydrates

• Blood types (ABO): Determined by specific oligosaccharides on red blood cell surfaces.

• Universal donor: Type O (no additional sugars).

• Universal acceptor: Type AB (has both A and B sugars).

• Heparin: A polysaccharide that acts as an anticoagulant (prevents blood clotting).