Organic Chemistry Structure, Bonding, and Acid-Base Chemistry Syllabus

How does a lone pair affect the molecular shape and bond angles compared with the electron-group geometry? Illustrate with water.

Lone pairs repel more strongly than bonding pairs, so they compress bond angles and the molecular shape (which counts only atoms) differs from electron geometry. Water ($\ce{H2O}$) has 4 electron groups (tetrahedral electron geometry) but a bent molecular shape with an angle of about $104.5^{\circ}$ (less than $109.5^{\circ}$).

What are the molecular shapes of $\ce{NH3}$ and $\ce{CO2}$, and their approximate bond angles?

$\ce{NH3}$: trigonal pyramidal, about $107^{\circ}$ (4 electron groups, one lone pair). $\ce{CO2}$: linear, $180^{\circ}$ (2 electron groups, no lone pairs on carbon).

How does bond order relate to bond length and bond strength?

As bond order increases (single → double → triple), bond length decreases and bond strength (bond dissociation energy) increases. Example: $\ce{C-C} > \ce{C=C} > \ce{C#C}$ in length, but the reverse order in strength.

How does hybridization (s-character) affect $\ce{C-H}$ bond length and $\ce{C-C}$ single-bond length?

Greater $s$-character holds bonding electrons closer to the nucleus, shortening and strengthening bonds. Order of $s$-character: $sp$ (50%) > $sp^{2}$ (33%) > $sp^{3}$ (25%), so $sp$ $\ce{C-H}$ bonds are shortest and $sp^{3}$ are longest.

What is the degree of unsaturation (index of hydrogen deficiency), and what does each degree represent?

The degree of unsaturation is the number of rings plus $\pi$ bonds in a molecule. Each degree corresponds to one ring or one double bond; a triple bond counts as 2 degrees.

Give the formula for calculating degrees of unsaturation for a compound containing C, H, N, O, and halogens.

$$\text{DoU} = \frac{2C + 2 + N - H - X}{2}$$ where $C$ = carbons, $H$ = hydrogens, $N$ = nitrogens, $X$ = halogens. Oxygen (and other divalent atoms) does not affect the count.

Calculate the degrees of unsaturation for benzene, $\ce{C6H6}$.

$$\text{DoU} = \frac{2(6) + 2 - 6}{2} = \frac{8}{2} = 4$$ This corresponds to 3 $\pi$ bonds + 1 ring, consistent with benzene.

Identify the functional groups: hydroxyl, carbonyl, carboxyl, and amino.

Hydroxyl: $\ce{-OH}$ (alcohols). Carbonyl: $\ce{C=O}$ (aldehydes/ketones). Carboxyl: $\ce{-COOH}$ (carboxylic acids). Amino: $\ce{-NH2}$ (amines).

How do you distinguish an aldehyde from a ketone by structure?

Both contain a carbonyl ($\ce{C=O}$). In an aldehyde the carbonyl carbon is bonded to at least one hydrogen and sits at the end of a chain ($\ce{-CHO}$). In a ketone the carbonyl carbon is bonded to two carbon groups ($\ce{R-CO-R'}$).

Describe the functional groups: ester, amide, and ether.

Ester: $\ce{-C(=O)O-}$ (carbonyl carbon bonded to an $\ce{-OR}$ group). Amide: $\ce{-C(=O)N-}$ (carbonyl carbon bonded to nitrogen). Ether: $\ce{R-O-R'}$ (oxygen single-bonded between two carbon groups).

State the basic steps of IUPAC nomenclature for a simple organic compound.

1) Find the longest continuous carbon chain containing the principal functional group (parent). 2) Number the chain to give the lowest locants to the principal group, then substituents. 3) Name and number substituents alphabetically. 4) Assemble: locants-prefixes + parent + suffix for the functional group.

What are the IUPAC parent-chain root names for chains of 1 through 6 carbons?

1: meth-, 2: eth-, 3: prop-, 4: but-, 5: pent-, 6: hex-.

What suffixes indicate an alkane, alkene, alkyne, alcohol, and carboxylic acid in IUPAC names?

Alkane: -ane; alkene: -ene; alkyne: -yne; alcohol: -ol; carboxylic acid: -oic acid.

When a chain has multiple substituents, how are locants and alphabetical order applied?

Number the chain so the set of locants is lowest at the first point of difference (with the principal functional group getting priority). List substituent prefixes alphabetically, ignoring multiplying prefixes (di-, tri-) when alphabetizing but keeping them in the name.

Give the IUPAC-preferred common class name and general suffix for compounds of type $\ce{R-OH}$, $\ce{R-NH2}$, and $\ce{R-CHO}$.

$\ce{R-OH}$: alcohol (suffix -ol). $\ce{R-NH2}$: amine (suffix -amine). $\ce{R-CHO}$: aldehyde (suffix -al).

What is the functional-group priority order (highest to lower) for choosing the principal characteristic group in naming?

A common order (highest first): carboxylic acid > ester > amide > nitrile > aldehyde > ketone > alcohol > amine > alkene/alkyne. The highest-priority group gets the suffix; lower groups become prefixes.

What are constitutional (structural) isomers?

Constitutional isomers are compounds with the same molecular formula but different connectivity — their atoms are bonded together in a different order (different skeletons, positions of functional groups, or functional group types).

Draw the distinction between the three types of constitutional isomerism.

Chain (skeletal) isomers: different carbon-chain branching (e.g. butane vs isobutane). Positional isomers: same skeleton but functional group in a different position (e.g. 1-propanol vs 2-propanol). Functional-group isomers: same formula, different functional group class (e.g. ethanol vs dimethyl ether, both $\ce{C2H6O}$).

State the Bronsted-Lowry definitions of an acid and a base.

A Bronsted-Lowry acid is a proton ($\ce{H+}$) donor; a Bronsted-Lowry base is a proton acceptor. Acid-base reactions involve transfer of a proton from acid to base.

What is a conjugate acid-base pair? Give an example.

A conjugate acid-base pair consists of two species differing by one proton. When an acid donates $\ce{H+}$ it becomes its conjugate base; when a base accepts $\ce{H+}$ it becomes its conjugate acid. Example: $\ce{HCl}$ / $\ce{Cl-}$ and $\ce{H2O}$ / $\ce{H3O+}$ in $\ce{HCl + H2O -> Cl- + H3O+}$.

Define $pK_a$ and state how it relates to acid strength.

$pK_a = -\log_{10} K_a$, where $K_a$ is the acid dissociation constant. A lower (or more negative) $pK_a$ means a stronger acid; a higher $pK_a$ means a weaker acid. A difference of one $pK_a$ unit corresponds to a factor of 10 in $K_a$.

In an acid-base equilibrium, how do you predict which side is favored using $pK_a$ values?

Equilibrium favors formation of the weaker acid and weaker base (higher $pK_a$). The proton moves from the stronger acid (lower $pK_a$) to the base that gives the more stable, weaker conjugate acid (higher $pK_a$).

List the main factors that stabilize a conjugate base and thus increase acid strength (the 'ARIO' factors).

Atom (electronegativity and size — larger, more electronegative atoms hold negative charge better), Resonance (delocalization stabilizes the charge), Induction (electron-withdrawing groups stabilize), and Orbital (more $s$-character stabilizes). A more stable conjugate base means a stronger acid.

Rank the acidity trend across a period and down a group of the periodic table for $\ce{H-A}$ bonds.

Across a period (left→right), acidity increases with electronegativity (e.g. $\ce{CH4} < \ce{NH3} < \ce{H2O} < \ce{HF}$). Down a group, acidity increases with atomic size / weaker $\ce{H-A}$ bond and more stable larger anion (e.g. $\ce{HF} < \ce{HCl} < \ce{HBr} < \ce{HI}$).

State the Lewis definitions of an acid and a base.

A Lewis acid is an electron-pair acceptor (has an empty orbital or can accept electrons). A Lewis base is an electron-pair donor (has a lone pair or $\pi$ electrons to donate). Lewis theory is broader than Bronsted-Lowry and does not require a proton.

Give two examples of Lewis acids that are not Bronsted acids.

$\ce{BF3}$ and $\ce{AlCl3}$ (electron-deficient with empty orbitals). Metal cations like $\ce{Fe^3+}$ or $\ce{Zn^2+}$ also act as Lewis acids. They accept electron pairs without donating a proton.

Define nucleophile and electrophile and relate them to Lewis acid-base terms.

A nucleophile ('nucleus-loving') is an electron-rich species that donates an electron pair to form a bond — it is a Lewis base. An electrophile ('electron-loving') is an electron-poor species that accepts an electron pair — it is a Lewis acid. Nucleophiles attack electrophiles.

What structural features make a species a good nucleophile, and how do nucleophilicity and basicity generally correlate?

Good nucleophiles have available lone pairs or $\pi$ electrons, negative or partial-negative charge, high polarizability, and low steric hindrance. Nucleophilicity often parallels basicity, but in protic solvents larger, more polarizable atoms (e.g. $\ce{I-}$) are better nucleophiles even if weaker bases, because of solvation effects.

What does a curved (curly) arrow represent in reaction mechanisms, and what does a full arrowhead vs a half (fishhook) arrowhead indicate?

A curved arrow shows the movement of electrons. A full (double-barbed) arrowhead shows movement of a pair of electrons (heterolytic/polar mechanisms). A half arrowhead (fishhook) shows movement of a single electron (radical/homolytic mechanisms).

State the rules for drawing curved arrows correctly in a polar mechanism.

Arrows always start from an electron source (a lone pair or a bond, i.e. electron-rich site) and point to an electron sink (an electron-poor atom or where a new bond forms / a bond breaks). Electrons flow from nucleophile to electrophile; the tail is at the electrons and the head is at their destination. Bookkeeping must conserve charge and never exceed an atom's octet.