(4503) CIE Topic 13 An Introduction to AS Level Organic Chemistry REVISION

Overview of Topic 13 – AS Organic Chemistry (CIE)

Imagine organic chemistry like building with special LEGOs! This first organic unit is super important because it teaches you all the basic rules for how these "LEGOs" (molecules) are put together. You'll learn how chemists draw molecules, group them into families (like different sets of LEGOs), give them special names so everyone knows exactly what they are, and understand why they react the way they do.

Mastering the special words and ideas here (like "formulae," "homologous series," "functional groups," "electrophile," "nucleophile," "radical") is like learning the alphabet before you can read a book. These words are essential for everything you'll learn later, right up to how chemists figure out what a mysterious substance is (Topic 22).

Types of Chemical Formulae

Think of a single molecule, like a specific type of LEGO model. You can describe it in a few different ways, and each way tells you something slightly different about it.

1. General formula

This is like a general recipe that fits a whole family of compounds. It's an algebraic expression that uses n to represent the number of carbon atoms. For example:

• For alkanes (a family of very simple carbon-hydrogen molecules, like straight chains of LEGOs with only single connections), the recipe is $C<em>nH</em>{2n+2}$. This means if you have 1 carbon ($n=1$), you'll have $(2 \times 1) + 2 = 4$ hydrogens, so $CH4$ (methane).

• For alkenes (another family, but they have at least one special "double connection" between carbons), the recipe is $C<em>nH</em>{2n}$. If you have 2 carbons ($n=2$), you'll have $(2 \times 2) = 4$ hydrogens, so $C2H4$ (ethene).

2. Molecular formula

This tells you the exact number of each atom in one molecule. It's like saying, "This LEGO model uses exactly 2 red bricks and 6 yellow bricks." For example, in ethane, the molecular formula is $C2H6$. This means there are exactly 2 carbon atoms and 6 hydrogen atoms in one molecule of ethane.

3. Empirical formula

This is the simplest, whole-number ratio of atoms in a compound. It's like simplifying a fraction. If you have a recipe that says "use 2 cups of sugar and 4 cups of flour," the simplest ratio is "1 cup of sugar to 2 cups of flour." For ethene, its molecular formula is $C2H4$. Both numbers (2 and 4) can be divided by 2, so the simplest ratio is $CH2$. This is its empirical formula.

4. Structural formula

This formula lists the atoms in the order they are connected, but without drawing all the bond lines. It's like writing down the sequence of your LEGO pieces: "red brick, then yellow brick, then blue brick." For example, butan-1-ol is written as $CH3CH2CH2CH2OH$. This tells you that a $CH3$ group is connected to a $CH2$ group, which is connected to another $CH2$ group, then another $CH2$ group, and finally an $OH$ group at the end.

5. Skeletal formula

This is a quick way to draw molecules, especially long ones. It only shows the carbon "backbone" (the main chain) and any atoms that aren't carbon or hydrogen (these are called "hetero-atoms" and are usually things like oxygen, nitrogen, or chlorine). You draw lines for the carbon chain, and:

• Every bend, corner, or end of a line is a carbon atom.

• We assume there are enough hydrogen atoms attached to each carbon to make it happy (carbon usually likes to have 4 connections). So, you don't draw the hydrogens on carbon.

• This makes it very fast to draw long chains because you don't have to draw every single $C–H$ bond.
  For example, a zigzag line might represent a chain of carbons. If you see a line ending, it's a $CH3$ group. If it's a bend in the middle, it's a $CH2$ group (unless it has other bonds to other carbons). If there's a carbon with three carbons attached, it's a $CH$ group.

6. Displayed formula

This is the most detailed drawing! It draws every single atom and every single covalent bond (the lines connecting atoms). When an examiner asks for a 'displayed' answer, you must draw all the bonds. For example, for 2-chloropropane, you would draw the three carbon atoms in a row, with a chlorine atom on the second carbon. Then, you would draw all the little lines that represent the carbon-hydrogen bonds, carbon-carbon bonds, and the carbon-chlorine bond.

Homologous Series

Imagine a family, like the "Smith Family." A homologous series is like a family of chemical compounds that are very similar but get a little bit longer or heavier as you go along. They share some key features:

1. They all have the same common functional group: This is a special atom or group of atoms that sits on the molecule and makes it act in a particular way. It's like every member of the Smith family might have a characteristic curly hair, which makes them recognizable.

2. They fit the same general formula: They all follow the same basic "recipe" or pattern, like the $C<em>nH</em>{2n+1}OH$ for alcohols.

3. Each member differs from the next by one methylene unit ($CH<em>2$): This is like adding one extra identical LEGO brick to the chain each time. So, if you have a molecule, the next one in the family is just that molecule plus a $CH</em>2$ group. For example, methane ($CH4$) to ethane ($C2H6$) adds a $CH2$ unit.

Let's look at the alcohol series as an example. Their general formula is $C<em>nH</em>{2n+1}OH$. You start with methanol ($CH3OH$), then add a $CH2$ to get ethanol ($CH3CH2OH$), then add another $CH2$ to get propan-1-ol ($CH3CH2CH2OH$), and so on to butan-1-ol, etc.

What's cool about this family is that their physical properties (like boiling point, how easily they evaporate) change smoothly and gradually as you add more $CH2$ units (they get bigger, so they stick together more). But their chemical reactivity (how they react with other chemicals) stays the same because they all have that special $-OH$ (alcohol) group, which is like their family's special skill!

IUPAC Nomenclature – Core Rules

"Nomenclature" is just a fancy word for "naming system." In chemistry, we use the IUPAC (International Union of Pure and Applied Chemistry) system to give every molecule a unique name so there's no confusion. It's like giving every house on a street a unique address.

Learning to name molecules might seem tricky at first, but with practice, it becomes easy, like riding a bike. Here's a step-by-step guide:

1. Identify the longest continuous carbon chain: First, find the longest continuous string of carbon atoms in the molecule without lifting your pencil. This chain determines the molecule's basic name, called the "stem." It's like finding the main street a house is on.

   • 1 carbon = meth–

   • 2 carbons = eth–

   • 3 carbons = prop–

   • 4 carbons = but–

   • 5 carbons = pent–

   • 6 carbons = hex–

   • 7 carbons = hept–

   • 8 carbons = oct–

   • 9 carbons = non–

   • 10 carbons = dec–

2. Locate and name the principal functional group: After finding the main carbon chain, look for the most important special group of atoms (your "functional group"). This group decides the ending, or "suffix," of the molecule's name. For example:

   • If it has only single carbon-carbon bonds (an alkane), it ends in -ane (e.g., methane).

   • If it has a carbon-carbon double bond (an alkene), it ends in -ene (e.g., ethene).

   • If it has an $-OH$ group (an alcohol), it ends in -ol (e.g., ethanol).

   • Other endings include -al (for aldehydes), -one (for ketones), and -oic acid (for carboxylic acids).

3. Number the chain: You need to give numbers to the carbon atoms in your longest chain. Start numbering from the end that gives your principal functional group the smallest possible number. It's like numbering houses on a street, but you start numbering from the end closest to the most important building (the functional group).

4. Insert that locant before the suffix: The number you just found for your functional group is called a "locant." You put this number right before the suffix to show where that important group is. For example, if the $-OH$ group is on the first carbon of a 4-carbon chain, it's butan-1-ol (not butan-4-ol).

5. Add side-chains or secondary functional groups as prefixes: If there are other smaller branches (called "side-chains") or other, less important, functional groups attached to your main carbon chain, you name them and add them to the beginning of the name. These are called "prefixes." You list them in alphabetical order. If you have more than one of the same kind of side-chain or group, you use multipliers like:

   • di- (for two)

   • tri- (for three)

   • tetra- (for four)
     For example, if you have two methyl groups, you'd call it "dimethyl."

Let's apply these rules to the example: 3-methylbutane-1,1-diol.

• Longest chain: "butane" tells us it's a 4-carbon chain (meth, eth, prop, but).

• Principal functional group: "-diol" tells us it has two $-OH$ (alcohol) groups (the "di" means two, and "ol" is for alcohol).

• Numbering and Locants: The "1,1" tells us both of those $-OH$ groups are on the first carbon atom of the 4-carbon chain.

• Side-chains: The "3-methyl" tells us there's a methyl group ($CH3$) attached to the third carbon atom of the main chain. So, it's a 4-carbon chain with two alcohol groups on the first carbon and a small $CH3$ branch on the third carbon.

Functional Groups Required at AS Level

These are the main types of special "handles" or features you need to know for your AS level. Each type makes the molecule behave in a unique way and gives it a specific ending or starting part of its name.

• Alkanes (-ane):

  • These are like very simple carbon chains where all the carbon atoms are connected by only single bonds ($C–C$). They are "saturated," meaning they have as many hydrogen atoms as they can hold, like a sponge full of water.

  • Example: methane, ethane, propane.

• Alkenes (-ene):

  • These molecules have at least one carbon-carbon double bond ($C=C$). This double bond is like having two strong connections between two specific carbon LEGO bricks.

  • Example: ethene, propene.

• Alcohols/Diols (-ol / hydroxy-):

  • These molecules contain an $-OH$ group (an oxygen atom connected to a hydrogen atom, which is then connected to a carbon). This $-OH$ group is what makes them "alcohols."

  • If a molecule has two $-OH$ groups, it's called a "diol."

  • Sometimes, if the $-OH$ group isn't the most important part, we call it a "hydroxy-" prefix.

  • Example: methanol, ethanol, propan-1-ol.