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Covers the systematic naming of complex organic compounds and the classification of structural isomers.
Did you know that two molecules can have the exact same chemical formula but act as completely different substances—one a life-saving medicine and the other a toxic compound? In organic chemistry, it’s not just about what atoms you have, but exactly where you put them.
Naming organic molecules isn't arbitrary; it follows a strict hierarchy established by IUPAC. To name a complex molecule, you must first identify the parent chain, which is the longest continuous carbon chain containing the highest-priority functional group. Priority follows a specific order: Carboxylic acids > Esters > Aldehydes > Ketones > Alcohols > Amines > Alkenes/Alkynes. Once the parent is found, number the chain to give the principal functional group the lowest possible number (the locant). Substituents like methyl () or ethyl () groups are listed alphabetically, regardless of their position number. For example, in a molecule with both a methyl group and a chlorine atom, 'chloro' comes before 'methyl' in the final name.
Name the following structure:
1. Identify the longest chain containing the triple bond: It has 6 carbons, so the parent is hexyne. 2. Number the chain to give the triple bond the lowest number: Numbering from the right gives the bond position 3. Numbering from the left also gives position 3. 3. Check substituents: There is a methyl group on carbon 5 (if numbering from the right) or carbon 2 (if numbering from the left). 4. Apply the lowest locant rule for substituents: We choose the numbering that gives the methyl group the number 2. 5. Final Name: 2-methylhex-3-yne.
Quick Check
In a molecule containing both a hydroxyl group () and a double bond (), which group determines the suffix and the numbering priority?
Answer
The hydroxyl group (-OH) has higher priority, so it determines the suffix (-ol) and receives the lowest possible locant.
Structural isomers (or constitutional isomers) share the same molecular formula but differ in the connectivity of their atoms. We classify them into three main types: 1. Chain Isomers: Differ in the arrangement of the carbon skeleton (e.g., straight vs. branched). 2. Positional Isomers: The carbon skeleton remains the same, but a functional group or substituent moves to a different carbon (e.g., propan-1-ol vs. propan-2-ol). 3. Functional Group Isomers: The atoms are rearranged to create entirely different functional groups. A classic example is , which can be ethanol (an alcohol) or methoxymethane (an ether). These isomers often have vastly different chemical reactivities.
Let's find three different types of structural isomers for :
1. Butan-1-ol: A straight-chain alcohol (). 2. Butan-2-ol: A positional isomer where the moves to the second carbon. 3. Ethoxyethane: A functional group isomer (), which is an ether. 4. 2-methylpropan-1-ol: A chain isomer of butan-1-ol where the carbon skeleton is branched.
Quick Check
What type of isomerism is shown by pentane and 2,2-dimethylpropane?
Answer
Chain isomerism, as they have the same formula () but different carbon skeleton arrangements.
The structure of an isomer directly influences its physical properties, particularly boiling point (BP) and solubility. For isomers with the same functional group, branching decreases the surface area of the molecule. This reduces the strength of London dispersion forces (intermolecular forces), leading to a lower BP. For example, pentane (linear) has a higher BP than neopentane (spherical/branched). Furthermore, functional group isomers show extreme differences; alcohols can form hydrogen bonds, giving them much higher BPs and better water solubility than their ether isomers, which only experience weaker dipole-dipole interactions.
Rank the following isomers of in order of increasing boiling point and justify your answer: (A) Diethyl ether, (B) Butan-1-ol, (C) 2-methylpropan-2-ol.
1. Lowest BP: (A) Diethyl ether. It is an ether and cannot form intermolecular hydrogen bonds. 2. Middle BP: (C) 2-methylpropan-2-ol. It is an alcohol (can hydrogen bond), but it is highly branched, which reduces surface area contact. 3. Highest BP: (B) Butan-1-ol. It is a primary alcohol with a straight chain, allowing for maximum surface area contact and strong hydrogen bonding.
What is the correct IUPAC name for a 5-carbon chain with a double bond at carbon 2 and a methyl group at carbon 4?
Which of the following pairs are functional group isomers?
Branching increases the boiling point of an organic compound because it allows molecules to pack more tightly.
Review Tomorrow
In 24 hours, try to draw all possible structural isomers for and from memory.
Practice Activity
Find a household product (like rubbing alcohol or nail polish remover), identify the active organic ingredient, and try to draw its structural isomers.