Organic Chemistry Notes (Unit 7)

0Where This Unit Fits

Up to now, chemistry has been about atoms and small molecules. Organic chemistry zooms in on one element, carbon, and the huge family of molecules it builds with hydrogen. Carbon makes 4 bonds, chains beautifully, and shows up in everything from gasoline to glucose.

I. Atomic Structure
II. The Periodic Table
III. Stoichiometry
IV. Chemical Bonding
V. Physical Behavior of Matter
VI. Equilibrium
VII. Organic Chemistry
VIII. Oxidation-Reduction
IX. Acids, Bases, and Salts
X. Nuclear Chemistry

By the end of this page you should be able to recognize an organic molecule, draw it four different ways, name alkanes with branches, identify constitutional isomers, name cycloalkanes, tell apart alkanes, alkenes, and alkynes, recognize the nine functional groups on Table R, and predict products of the seven organic reactions on the exam.

1What Makes a Molecule Organic?

An organic molecule is a molecule built mostly out of carbon and hydrogen. Carbon sits in group 4 (the 4th family) on the periodic table, so it needs 4 bonds to fill its octet, with 0 lone pairs.

    Carbon's bonding rules:
    
· Group 4 → needs 4 electrons → forms 4 bonds, 0 lone pairs.
    
· Single bond = 1 sigma ($\sigma$) bond.
    
· Double bond = 1 sigma + 1 pi ($\pi$) bond.
    
· Triple bond = 1 sigma + 2 pi bonds.
    
    Hydrogen always makes exactly 1 bond.

Hydrocarbons

A hydrocarbon is a molecule that contains only carbon and hydrogen, with C−C and C−H bonds. They split into two big classes:

Alkane

C−C

All single bonds. Saturated. Aliphatic.

Alkene

C=C

At least one double bond. Unsaturated. Aliphatic.

Alkyne

C≡C

At least one triple bond. Unsaturated. Aliphatic.

Arene

◯

Aromatic ring (like benzene). Special class.

Saturated means every C−C bond is single. The molecule is "full" of hydrogens. Unsaturated means there is at least one double or triple bond, so the molecule has fewer hydrogens than a saturated one with the same number of carbons.

2Four Ways to Draw a Molecule

The same molecule can be written in multiple ways. They all describe the same compound, but they hide or show different details. Knowing how to switch between them is half the battle.

Molecular formulaJust the count of each atom. Hides connectivity. e.g. $\ce{C4H10}$. Structural formulaEvery atom and every bond shown explicitly. Bulky but clear. Condensed formulaA linear text version. Bonds are implied. e.g. CH3CH2$\ce{CH2}$$\ce{CH3}$. Skeletal (line-angle)Just the carbon skeleton as zigzag lines. Carbons are at ends and joints. Hydrogens on carbon are not drawn.

Compare All Four Representations of n-Butane

Molecular formula

$\ce{C4H10}$

Just the atom counts. Tells you what is in the molecule, not how it is connected.

3D Cues in 2D Drawings

Real molecules are 3D, but paper is flat. We use three bond styles to fake depth:

Plain line

In the plane of the page.

Solid wedge

Comes out of the page, toward you.

Dashed wedge

Goes behind the page, away from you.

3Alkanes: The Simplest Hydrocarbons

An alkane is a hydrocarbon where every C−C bond is a single bond. The name always ends in -ane. Alkanes are the backbone family. Almost everything else is built off them by adding a feature.

    Alkane formula: $\ce{C_nH_{2n+2}}$
    
where $n$ is the number of carbons

The first part of an alkane's name comes from a prefix table that tells you how many carbons are in the chain. Click any prefix to see the matching name and formula.

Carbon Count → Name + Formula

Carbons (n):

Prefix

but

Alkane

butane

Alkene

butene

Alkyne

butyne

Formula (alkane)

$\ce{C4H10}$

Three Group Names to Memorize

methyl$\ce{CH3}$ — a carbon with 3 hydrogens, bonded to one other thing. Always at the end of a chain or as a branch. methylene$\ce{CH2}$ — a carbon with 2 hydrogens, bonded to two other carbons. The repeating "middle" piece in a chain. methineCH — a carbon with 1 hydrogen, bonded to three other carbons. Shows up at branch points.

A straight-chain alkane contains only methyl groups (the two ends) and methylene groups (everything between). The general condensed formula is $\ce{CH3(CH2)_xCH3}$. This family is called the homologous series — each member differs from the next by exactly one $\ce{CH2}$.

4Isomers: Same Atoms, Different Layout

Constitutional isomers are two molecules with the same molecular formula but a different structure. They have different names, different condensed formulas, and usually different physical properties.

    Same molecular formula. Different connectivity.
    
    Both have $\ce{C4H10}$, but the way the atoms are arranged is different. That makes them isomers.

n-Butane vs Isobutane — Same $\ce{C4H10}$, Different Structure

n-butane (straight)

$\ce{CH3CH2CH2CH3}$
4 carbons

isobutane (2-methylpropane)

$\ce{(CH3)3CH}$
4 carbons, branched

Carbon Classification: 1°, 2°, 3°, 4°

Each carbon in an alkane gets a label based on how many other carbons it is bonded to.

primary (1°)Bonded to one other carbon. The end of a chain. secondary (2°)Bonded to two other carbons. Middle of a straight chain. tertiary (3°)Bonded to three other carbons. A branch point. quaternary (4°)Bonded to four other carbons. Branch point with no hydrogens.

Click a Carbon to Classify It

1° primary 2° secondary 3° tertiary 4° quaternary

Tap any carbon vertex to see its classification.

The prefixes n- (normal, no branching) and iso- (one methyl branch creating a methine carbon) are old common names. Modern naming gives every isomer a unique systematic name instead. We learn the systematic rules next.

5Naming an Alkane: Step-by-Step Walkthrough

Click Next to walk through every rule on a worked example. Each step highlights the part of the molecule the rule is about.

Two rules students miss most

Number the longest chain so the branches get the lowest possible numbers. Always check both directions and pick the one that gives the lowest set of locants at the first point of difference.
Count prefixes (di-, tri-, tetra-, etc.) in front of branch names DO NOT count when alphabetizing. "dimethyl" alphabetizes under m, not d. Same for "triethyl" under e, and so on. The hyphenated prefixes tert- and sec- are also ignored when alphabetizing.

Naming Walkthrough

Step 0 / 7

Click Next to start. We will name this molecule one rule at a time.

6Branches and Complex Branches

A branch (also called a substituent) is anything sticking off the parent chain that is not part of it. Simple branches like methyl ($\ce{CH3-}$) and ethyl (CH3CH2−) are easy. Bigger branches can themselves have branches — those need parentheses and an internal locant.

    Just like a carbon can be 1°, 2°, 3°, or 4°, an alkyl branch can be 1° or 2° depending on which kind of carbon it attaches through.
    
    When the branch itself has branches, write the branch name in parentheses with its own internal numbering.

Common-Name Shortcuts to Memorize

Five branched alkyl groups have short common names that show up everywhere. Knowing them on sight saves time. The systematic name is in parentheses.

isopropyl

(1-methylethyl)

$\ce{(CH3)2CH-}$

sec-butyl

(1-methylpropyl)

$\ce{CH3CH2CH(CH3)-}$

isobutyl

(2-methylpropyl)

$\ce{(CH3)2CHCH2-}$

tert-butyl

(1,1-dimethylethyl)

$\ce{(CH3)3C-}$

neopentyl

(2,2-dimethylpropyl)

$\ce{(CH3)3CCH2-}$

The asterisk (*) marks the connection point where the branch attaches to the parent chain.

    Alphabetizing rule: when sorting branches alphabetically, treat iso- and neo- as part of the name (they count). But hyphenated prefixes like tert- and sec- are ignored when alphabetizing. tert-butyl files under b.
  

7Cycloalkanes: Carbon Chains in a Ring

A cycloalkane is a hydrocarbon where the carbon chain closes back on itself to form a ring. The naming follows the same rules as a chain alkane, with two twists: the prefix cyclo- goes in front of the parent name, and the locant rules for substituents are slightly different.

    Locant rules for cycloalkanes:
    
· Mono-substituted cycloalkane (just one branch) → no locant needed. Example: methylcyclohexane.
    
· Di-substituted (or more) → locants required. Number to give the lowest set, regardless of branch identity.
    
· A chain alkane with one branch still needs a locant (the chain is not symmetric).
    
· When you have a chain attached to a ring, the parent is whichever has more carbons.

Cycloalkane Numbering: Mono vs Di-Substituted

A few cycloalkanes worth recognizing on sight: cyclopropane (3-carbon triangle), cyclobutane (square), cyclopentane (pentagon), cyclohexane (hexagon). Cyclohexane is by far the most common ring in organic chemistry.

8Alkenes and Alkynes: Multiple Bonds

When a hydrocarbon has a double or triple bond between carbons, it is unsaturated. The suffix changes to reflect the bond type:

    -ane → saturated, all single bonds (alkane).
    
    -ene → at least one double bond (alkene).
    
    -yne → at least one triple bond (alkyne).

Same 3-Carbon Skeleton, Three Saturation States

For alkenes and alkynes, when you number the parent chain, the multiple bond gets the lowest possible number. The locant for the multiple bond goes right before the suffix. e.g. but-2-ene means the double bond starts at carbon 2.

Example. $\ce{CH3-CH=CH-CH3}$.

Parent. 4 carbons → but-.

Suffix. One double bond → -ene.

Locant. Number from the end giving the double bond the lowest number. The C=C starts at carbon 2.

Final name. but-2-ene (also written 2-butene).

9Functional Groups: Table R

A functional group is a small arrangement of atoms (often containing oxygen, nitrogen, or a halogen) that gives a molecule a characteristic set of properties. Swap a hydrogen on a hydrocarbon for a functional group, and you change the chemistry. Table R on the Reference Tables lists the nine families you need to recognize.

    How to read Table R. For each family, learn three things:
    
· The functional group (the cluster of atoms).
    
· The suffix or prefix added to the name.
    
· A simple example you can recognize on sight.

1. Browse: tap a family to see its group, suffix, example, and behaviour

Watch out

Aldehyde vs ketone. Both have $\ce{C=O}$ (a carbonyl group), but an aldehyde has the carbonyl on a terminal carbon (with an H attached), and a ketone has it on a carbon between two other carbons (no H on the carbonyl carbon).
Carboxylic acid vs ester. A carboxylic acid ends in $\ce{-COOH}$ (the OH on the carbonyl). In an ester, that OH is replaced by an $\ce{-OR}$ group (an oxygen-carbon).
Amine vs amide. An amine is just $\ce{-NH2}$ on a carbon. An amide is $\ce{-CONH2}$ — there is a carbonyl right next to the nitrogen.

2. Build & name: pick a carbon backbone and a functional group

The same carbon skeleton (the parent chain) takes a different name depending on which functional group is attached. Try the combinations to see how the suffix changes.

Carbon backbone

Functional group

3. Practice: identify the group, then name the molecule

Question 1 / 8

Score: 0

10Seven Types of Organic Reactions

Organic molecules react in a small number of predictable ways. The Regents expects you to recognize seven reaction types and to predict the products of each.

    Quick-recognition rules.
    
· If a $\ce{C=C}$ double bond goes away and a new atom or group gets added across it → addition.
    
· If a hydrogen on a saturated alkane is replaced by another atom (usually a halogen) → substitution.
    
· If many small alkene units link into one long chain → polymerization (specifically, addition polymerization).
    
· If an alcohol plus a carboxylic acid produces an ester plus water → esterification.
    
· If a sugar (glucose) breaks down into ethanol plus carbon dioxide using yeast → fermentation.
    
· If a hydrocarbon plus oxygen produces carbon dioxide plus water → combustion.
    
· If a fat (an ester) plus a strong base (like NaOH) produces soap plus glycerol → saponification.

Pick a Reaction. See the Equation, Then Play the Animation

11See It in 3D

Skeletal structures are flat, but real molecules are 3D. The zigzag in a hydrocarbon chain isn't artistic license — it shows that carbon is tetrahedral, with bond angles of about 109.5°. Drag any model below to rotate, scroll to zoom.

Hydrocarbon chains: pick a molecule

Carbon is tetrahedral — every C−C−C angle is roughly 109.5°. The zigzag in a 2D drawing is a flat shadow of that 3D geometry. Try cyclohexane to see the famous "chair" shape.

Stereochemistry: (R) vs (S)-2-bromobutane

When a carbon has four different substituents (a stereocenter), the molecule has two non-superimposable mirror-image forms. The wedges and dashes encode which one. Rotate each model below to see how the wedge means "out of the page" in real 3D.

(R)

Bromine, methyl, ethyl, and H attached to one carbon. Priority order Br > ethyl > methyl > H rotates clockwise = R.

(S)

Same atoms, mirror image. Priority order rotates counter-clockwise = S. Hold them up to a mirror in your head — they cannot be superimposed.

12Flashcards: Self-Quiz Yourself

Click the card to flip. Use the arrows to move through the deck. Shuffle whenever you want a fresh order.

term tap to reveal

definition tap to flip back

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13Naming Practice Quiz

Look at the structure, pick the correct name. Click Next to advance after each answer.

Question 1 / 7

Score: 0

What is the correct name for this molecule?

14Practice Problems

Try each one before opening the answer.

1. An alkane has 7 carbons. What is its name and molecular formula?

Prefix for 7 carbons is hept-. Suffix for an alkane is -ane. Name: heptane. Formula: $\ce{C_nH_{2n+2}}$ with $n=7$ gives $\ce{C7H16}$.

2. Which pair below shows constitutional isomers?
(A) $\ce{C2H6}$ and $\ce{C3H8}$
(B) n-butane and isobutane
(C) methane and ethene
(D) propane and propene

(B). Both n-butane and isobutane have the same molecular formula ($\ce{C4H10}$) but different structures (straight vs branched). That is the definition of constitutional isomers. (A) different formulas. (C) different formulas, different families. (D) different formulas ($\ce{C3H8}$ vs $\ce{C3H6}$).

3. Name the molecule: $\ce{CH3-CH(CH3)-CH2-CH2-CH3}$.

Longest chain: 5 carbons → pentane. Number from the end closest to the branch: the methyl branch is on carbon 2. Name: 2-methylpentane.

4. What is the difference between an alkene and an alkyne?

An alkene has at least one carbon-carbon double bond (C=C) and ends in -ene. An alkyne has at least one carbon-carbon triple bond (C≡C) and ends in -yne. Both are unsaturated hydrocarbons.

5. An alkane has 12 hydrogens. How many carbons does it have?

Use $\ce{C_nH_{2n+2}}$. Set $2n+2 = 12$, so $n = 5$. The alkane is pentane, $\ce{C5H12}$.

6. A carbon is bonded to three other carbons and one hydrogen. Classify it.

Bonded to 3 other carbons makes it a tertiary (3°) carbon. The single H confirms it — this is a methine carbon.

7. An alkane has the structure: a 5-carbon chain with a methyl branch on C2 and an ethyl branch on C3. What is its name?

Parent: pentane. Branches: 2-methyl, 3-ethyl. Alphabetize (ethyl before methyl): 3-ethyl-2-methylpentane. Notice "ethyl" comes first because e is before m.

8. What does the prefix cyclo- tell you about a hydrocarbon?

The carbon chain forms a closed ring instead of an open chain. For example, cyclohexane is a 6-carbon ring (a hexagon).

9. In a cyclopentane with one methyl group attached, do you need to give the methyl a locant number? Why or why not?

No locant needed. When a cycloalkane has only one substituent, the molecule is symmetric and any starting point gives the same name. The compound is just methylcyclopentane. Locants are required only when there are 2 or more substituents.

10. Which name is correct for an alkane with two methyl branches both on carbon 2 of butane?
(A) 2,2-methylmethylbutane
(B) 2,2-dimethylbutane
(C) 2-dimethylbutane
(D) bismethyl-2-butane

(B) 2,2-dimethylbutane. When the same branch repeats, condense with the prefix di- (or tri-, tetra-, etc.) and stack the locants separated by commas. Each methyl still needs its own locant, even when they share a carbon.

11. Draw or describe the difference in structural formula between propene and propyne.

Propene ($\ce{C3H6}$): $\ce{CH2=CH-CH3}$, has a C=C double bond. Propyne ($\ce{C3H4}$): $\ce{CH#C-CH3}$, has a C≡C triple bond. Same 3-carbon skeleton, different bond order at the front, different number of hydrogens.

12. The common name isopropyl corresponds to which systematic branch name?

(1-methylethyl). Isopropyl is a 3-carbon branch attached through the middle carbon: $\ce{(CH3)2CH-}$.

13. Name the functional group present in each compound and the family it belongs to:
(a) $\ce{CH3CH2OH}$
(b) $\ce{CH3COOH}$
(c) $\ce{CH3CHO}$
(d) $\ce{CH3OCH3}$

(a) $\ce{-OH}$ on a carbon → alcohol (ethanol).
(b) $\ce{-COOH}$ → organic acid / carboxylic acid (ethanoic acid).
(c) $\ce{-CHO}$ at the end → aldehyde (ethanal).
(d) Oxygen between two carbons → ether (dimethyl ether).

14. What is the difference between an aldehyde and a ketone? Give an example of each.

Both have a carbonyl ($\ce{C=O}$). In an aldehyde, the carbonyl is on a terminal carbon, so that carbon also has a hydrogen ($\ce{R-CHO}$). Example: ethanal, $\ce{CH3CHO}$. In a ketone, the carbonyl is on an internal carbon between two other carbons ($\ce{R-CO-R'}$). Example: propanone (acetone), $\ce{(CH3)2CO}$.

15. Predict the product. Ethene ($\ce{CH2=CH2}$) reacts with bromine ($\ce{Br2}$). What type of reaction is this, and what is the product?

This is an addition reaction — the $\ce{Br2}$ molecule splits and adds across the C=C double bond. Product: 1,2-dibromoethane, $\ce{CH2Br-CH2Br}$. The double bond becomes a single bond. (Lab clue: the orange bromine colour fades to colourless, confirming the alkene is unsaturated.)

16. Methane ($\ce{CH4}$) reacts with chlorine ($\ce{Cl2}$) in the presence of UV light. What type of reaction is this, and what are the products?

This is a substitution reaction — a hydrogen on the alkane is replaced by a chlorine atom. Products: chloromethane ($\ce{CH3Cl}$) + hydrogen chloride ($\ce{HCl}$). Substitution requires saturated carbons and (usually) UV light or heat.

17. Ethanoic acid ($\ce{CH3COOH}$) reacts with methanol ($\ce{CH3OH}$) in the presence of a sulfuric acid catalyst. Name the reaction type and predict the products.

This is esterification. An organic acid + an alcohol gives an ester + water. The product ester is methyl ethanoate ($\ce{CH3COOCH3}$), plus water ($\ce{H2O}$). The $\ce{-OH}$ from the acid combines with the $\ce{H}$ from the alcohol to form the water.

18. Glucose ($\ce{C6H12O6}$) is broken down by yeast. Write the balanced reaction and name the process.

This is fermentation:
$\ce{C6H12O6 ->[\text{yeast}] 2\,C2H5OH + 2\,CO2}$
One glucose molecule produces two ethanol molecules + two $\ce{CO2}$. The reaction is anaerobic (no oxygen needed). This is how bread rises and how alcoholic beverages are produced.

19. Balance the combustion of propane ($\ce{C3H8}$) in excess oxygen. What type of reaction is this?

This is combustion. A hydrocarbon plus $\ce{O2}$ gives $\ce{CO2}$ + $\ce{H2O}$. Balance carbons first (3 $\ce{CO2}$), then hydrogens (4 $\ce{H2O}$), then count oxygens on the right (6 + 4 = 10) and put $\frac{10}{2} = 5$ in front of $\ce{O2}$:
$\ce{C3H8 + 5\,O2 -> 3\,CO2 + 4\,H2O}$
This reaction is highly exothermic — it is why propane is used as a fuel.

20. What is saponification, and why is it called "soap-making"?

Saponification is the reaction between a fat (an ester of glycerol with three long-chain carboxylic acids) and a strong base (NaOH). The base breaks each ester bond, producing three sodium-carboxylate molecules (soap) + glycerol (a 3-carbon alcohol with three OH groups). It is literally how soap is made — the Latin word sapo means soap. It is the reverse of esterification, run with a base instead of an acid.

15Quick Review Cheat Sheet

Organic moleculeBuilt from carbon and hydrogen, sometimes with extra elements. HydrocarbonOnly C and H. C−C and C−H bonds. SaturatedAll single bonds. The molecule is "full" of hydrogens. UnsaturatedHas at least one double or triple bond. AlkaneAll single bonds. Suffix -ane. Formula $\ce{C_nH_{2n+2}}$. AlkeneAt least one C=C. Suffix -ene. AlkyneAt least one C≡C. Suffix -yne. CycloalkaneRing of carbons. Prefix cyclo-. Constitutional isomerSame molecular formula, different connectivity. Methyl / methylene / methine$\ce{CH3}$ / $\ce{CH2}$ / CH groups. 1° / 2° / 3° / 4°Carbon classified by how many other carbons it is bonded to. Branch / substituentAnything sticking off the parent chain. Lowest locant ruleNumber the parent so branches get the lowest possible numbers (first point of difference). Alphabetize branchesIgnore prefix counts (di-, tri-) and hyphenated prefixes (tert-, sec-) when sorting. Skeletal structureCarbon at every end and joint, hydrogens not drawn. isopropyl$\ce{(CH3)2CH-}$ = (1-methylethyl). sec-butyl$\ce{CH3CH2CH(CH3)-}$ = (1-methylpropyl). isobutyl$\ce{(CH3)2CHCH2-}$ = (2-methylpropyl). tert-butyl$\ce{(CH3)3C-}$ = (1,1-dimethylethyl). neopentyl$\ce{(CH3)3CCH2-}$ = (2,2-dimethylpropyl).

Functional Groups (Table R)

Halide$\ce{R-X}$ (X = F, Cl, Br, I). Prefix halo-. e.g. chloromethane. Alcohol$\ce{R-OH}$. Suffix -ol. e.g. ethanol. Ether$\ce{R-O-R'}$. e.g. dimethyl ether ($\ce{CH3OCH3}$). Aldehyde$\ce{R-CHO}$ (C=O on terminal C). Suffix -al. e.g. ethanal. Ketone$\ce{R-CO-R'}$ (C=O on internal C). Suffix -one. e.g. propanone (acetone). Organic acid$\ce{R-COOH}$. Suffix -oic acid. e.g. ethanoic acid (vinegar). Ester$\ce{R-COO-R'}$. Suffix -yl ...oate. Sweet smelling. e.g. methyl ethanoate. Amine$\ce{R-NH2}$. Suffix -amine. Weakly basic. e.g. methylamine. Amide$\ce{R-CONH2}$ (N next to a carbonyl). e.g. ethanamide. Amino acidHas both $\ce{-NH2}$ and $\ce{-COOH}$. e.g. glycine.

Seven Organic Reactions

Addition$\ce{C=C + X-Y -> C(X)-C(Y)}$. Needs an alkene or alkyne. Test for unsaturation: $\ce{Br2}$ decolourises. Substitution$\ce{R-H + X2 -> R-X + HX}$. Needs an alkane + UV light. PolymerizationMany monomers (alkenes) link end-to-end into a polymer. e.g. ethene → polyethylene. EsterificationAcid + alcohol $\rightarrow$ ester + water. Needs $\ce{H2SO4}$ catalyst. Fermentation$\ce{C6H12O6 ->[\text{yeast}] 2\,C2H5OH + 2\,CO2}$. Anaerobic. CombustionHydrocarbon + $\ce{O2} \rightarrow \ce{CO2} + \ce{H2O}$. Exothermic. Balance carbons, then hydrogens, then oxygens. SaponificationFat + NaOH $\rightarrow$ soap + glycerol. The reverse of esterification.