Chapter 4: Representations and Properties of Organic Molecules

Representing Molecules

Organic molecules can be represented in several ways, each providing different levels of structural detail and utility for chemical analysis.

• Molecular Formula: Shows the number and type of atoms (e.g., C2H6 for ethane).

• Condensed Formula: Groups atoms to show connectivity (e.g., CH3CH3).

• Lewis/Expanded Structure: Displays all atoms and bonds, including lone pairs.

• Skeletal Structure: Simplifies organic molecules by representing carbon atoms as vertices and omitting hydrogens bonded to carbons.

Example: Ethanol can be represented as C2H6O (molecular), CH3CH2OH (condensed), or with a full Lewis structure showing all bonds and lone pairs.

Functional Group Identification

Functional groups are specific groups of atoms within molecules that determine characteristic chemical reactions.

• Alkanes: Saturated hydrocarbons (single bonds only).

• Alkenes: Hydrocarbons with at least one double bond.

• Alkynes: Hydrocarbons with at least one triple bond.

• Alcohols: Contain an -OH group.

• Amine: Contain an -NH2, -NHR, or -NR2 group.

• Ether: Contain an R-O-R' linkage.

• Alkyl Halide: Contain a halogen (F, Cl, Br, I) attached to an alkyl group.

• Aromatic: Contain a benzene ring (non-carbonyl).

• Amides, Carboxylic Acids, Aldehydes, Ketones, Esters: All contain a carbonyl group (C=O) with different substituents.

Example: Acetone is a ketone; ethanol is an alcohol.

IUPAC Naming and Substituent Naming

• IUPAC Naming: Systematic method for naming organic compounds based on the longest carbon chain and functional groups.

• Alkyl Substituents: Methyl (CH3-), ethyl (C2H5-), propyl (C3H7-), etc.

• Halogen Substituents: Chloro-, bromo-, fluoro-, iodo-.

Example: 2-bromopropane: a propane chain with a bromine on the second carbon.

Bond Length vs. Bond Strength

• Bond Length: The distance between the nuclei of two bonded atoms.

• Bond Strength: The energy required to break a bond; generally, shorter bonds are stronger.

Example: Triple bonds (alkynes) are shorter and stronger than double (alkenes) or single (alkanes) bonds.

Isomerism

• Structural Isomers: Same molecular formula, different connectivity.

• Stereoisomers: Same connectivity, different spatial arrangement.

• Cis/Trans Isomers: Stereoisomers with different positions around a double bond or ring.

• Chiral Molecules: Molecules that are non-superimposable on their mirror images; contain a chiral center (usually a carbon with four different groups).

• Enantiomers: Pairs of chiral molecules that are mirror images.

Example: 2-butanol has two enantiomers (R and S forms).

Fatty Acid Nomenclature

• Omega (ω) Designation: Indicates the position of the first double bond from the methyl end.

• X:X Designation: Number of carbons:number of double bonds (e.g., 18:1).

• Saturated: No double bonds.

• Monounsaturated: One double bond.

• Polyunsaturated: More than one double bond.

Example: Oleic acid is 18:1 ω-9 (18 carbons, 1 double bond at the 9th carbon from the methyl end).

Extra Credit Functional Groups

• Thiols: Contain -SH group.

• Sulfides: Contain R-S-R' linkage.

• Disulfides: Contain R-S-S-R' linkage.

• Phenols: Aromatic ring with an -OH group.

Chapter 5: Thermochemistry and Organic Reactions

Enthalpy (ΔH) and Free Energy (ΔG)

Thermochemistry studies the energy changes in chemical reactions, focusing on enthalpy and free energy.

• Enthalpy (ΔH): Heat change at constant pressure.

• Exothermic: ΔH < 0; releases heat.

• Endothermic: ΔH > 0; absorbs heat.

• Free Energy (ΔG): Determines spontaneity of a reaction.

• Exergonic: ΔG < 0; spontaneous.

• Endergonic: ΔG > 0; non-spontaneous.

Equation:

Example: Combustion of methane is exothermic and exergonic.

Increasing Reaction Rate

• Increase temperature.

• Increase concentration of reactants.

• Add a catalyst to lower activation energy.

Categories and Types of Organic Reactions

• Synthesis: Two or more reactants combine to form one product.

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

• Exchange/Displacement: Atoms or groups are exchanged between molecules.

Specific Types:

• Redox (Inorganic and Organic): Involves transfer of electrons or changes in oxidation state.

• Combustion: Reaction with O2 producing CO2 and H2O.

• Addition Reactions: Atoms added to double or triple bonds (e.g., hydration, hydrogenation).

• Condensation: Two molecules combine, releasing a small molecule (often H2O).

• Hydrolysis: Breaking a bond using water.

Example: Hydration of ethene forms ethanol.

Chapter 6: Carbohydrates and Their Chemistry

Classification of Carbohydrates

• Monosaccharides: Single sugar units (e.g., glucose).

• Disaccharides: Two monosaccharides joined (e.g., sucrose).

• Oligosaccharides: 3–10 monosaccharide units.

• Polysaccharides: Many monosaccharide units (e.g., starch, cellulose).

Alcohol Classification in Carbohydrates

• Primary Alcohol: -OH on a carbon attached to one other carbon.

• Secondary Alcohol: -OH on a carbon attached to two other carbons.

• Tertiary Alcohol: -OH on a carbon attached to three other carbons.

Monosaccharide Structure and Isomerism

• Triose, Tetrose, Pentose, Hexose: 3, 4, 5, or 6 carbon sugars, respectively.

• Aldose: Monosaccharide with an aldehyde group.

• Ketose: Monosaccharide with a ketone group.

• Diastereomers: Stereoisomers not related as mirror images.

• Enantiomers: Non-superimposable mirror images.

• Epimers: Differ at only one chiral center.

Anomeric Carbon and Cyclic Forms

• Anomeric Carbon: The carbon derived from the carbonyl carbon (C1 in aldoses) that becomes chiral in the cyclic form.

• Fischer to Cyclic (Hemiacetal) Form: The anomeric carbon bonds to an -OH group, forming a ring.

• Alpha (α) and Beta (β) Anomers: If the -OH on the anomeric carbon is down (relative to the ring), it is α; if up, it is β.

Example: Glucose forms α-D-glucopyranose and β-D-glucopyranose.

Reducing Sugars

• Monosaccharides and some disaccharides can reduce mild oxidizing agents due to a free anomeric carbon.

• Reducing sugars have a free -OH on the anomeric carbon.

Glycosidic Bonds and Disaccharide Formation

• Two monosaccharides join via condensation, forming a glycosidic bond (α or β, depending on the orientation).

• Bond is named by the carbons involved (e.g., α(1→4)).

Polysaccharides: Storage and Structure

• Storage Polysaccharides: Starch (plants: amylose and amylopectin), glycogen (animals).

• Structural Polysaccharides: Cellulose (plants), chitin (arthropods, fungi).

• Amylose: Unbranched α(1→4) glucose polymer.

• Amylopectin: Branched α(1→4) and α(1→6) glucose polymer.

• Cellulose: Unbranched β(1→4) glucose polymer.

• Chitin: Similar to cellulose but with N-acetylglucosamine units.

• Glycogen: Highly branched α(1→4) and α(1→6) glucose polymer.

Biological Applications

• Starch: Plant glucose storage (amylose + amylopectin).

• ABO Blood Typing: Oligosaccharide antigens determine blood type; O is universal donor, AB is universal acceptor.

• Heparin: A polysaccharide anticoagulant that prevents blood clotting.

Extra Credit: Projections

• Fischer to Haworth: Converting linear to cyclic forms.

• Haworth to Fischer: Converting cyclic to linear forms.

Chapter 7.1: Gas Laws and Pressure Units

Pressure Units and Conversions

Pressure is a fundamental property of gases, measured in various units. Common units include atmospheres (atm), millimeters of mercury (mmHg), torr, and pascals (Pa).

• 1 atm = 760 mmHg = 760 torr = 101,325 Pa

Example: To convert 2 atm to mmHg: 2 atm × 760 mmHg/atm = 1520 mmHg.

Gas Laws (Conceptual)

• Boyle’s Law: At constant temperature, pressure and volume are inversely related.

• Charles’ Law: At constant pressure, volume and temperature are directly related.

• Gay-Lussac’s Law: At constant volume, pressure and temperature are directly related.

Equations:

• Boyle’s Law:

• Charles’ Law:

• Gay-Lussac’s Law:

Example: If a gas at 1.0 atm and 300 K is heated to 600 K at constant volume, its pressure doubles to 2.0 atm (Gay-Lussac’s Law).

Extra Credit

• Use the above equations to solve for unknown variables when given three of the four quantities.