Functional Groups

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    Chemical molecules are similar. Just because they are made from the same number and type of atoms, it doesn't mean that they are similar in any other way. One might readily react with water or acid, whilst the other might make a great fuel. A molecule's characteristics, properties, and typical reactions are all governed by its arrangement of atoms into specific functional groups.

    • This article is about functional groups in organic chemistry.
    • We'll define functional groups, then provide you with a functional group table.
    • After that, we'll look at examples of functional groups.
    • We'll end with a couple of functional group practice questions.

    Functional groups definition

    Functional groups are specific groups of atoms that are responsible for a molecule's characteristic reactions and properties.

    Alkenes, alcohols, and carboxylic acids - these are all types of organic molecules. In particular, they are examples of homologous series (also known as families). Molecules within a homologous series react in a very similar way to each other, and this is because they all contain the same particular collection of atoms - their functional group. Members of a homologous series may have different lengths or arrangements of their carbon chain but each one of them features the same functional group at some point within their structure. This means that they take part in the same general reactions.

    Functional groups are useful because they allow us to predict the characteristics of an unknown molecule just by looking at its structure. If we identify its functional group, we can compare it to other molecules in the same homologous series or family and make confident guesses as to how it reacts.

    Functional groups table

    Let's find out about various different functional groups you'll encounter in organic chemistry with the help of a handy functional groups table. This table names the functional group associated with specific organic families, along with the functional group formula and an example molecule.

    Family nameFunctional group nameFunctional group formulaExample
    AlkaneC-C single bondC-CEthane
    AlkeneC=C double bondC=CEthene
    AlkyneC≡C triple bondC≡CEthyne
    AlcoholHydroxyl-OHMethanol
    HalogenoalkaneHalogen atom-XChloromethane
    AldehydeCarbonyl (aldehyde)-CHOPropanal
    KetoneCarbonyl (ketone)-CO-Propanone
    Carboxylic acidCarboxyl-COOHEthanoic acid
    Acyl chlorideAcyl chloride-COClEthanoyl chloride
    EsterEster link-COO-Methyl ethanoate
    AmineAmine-NH2Methylamine
    AmideAmide-CONH2Ethanamide
    AreneAreneC6H6Benzene
    HalogenoarenePhenylHalogen atomC6H5XChlorobenzene
    PhenolPhenylHydroxylC6H5OHPhenol (hydroxybenzene)

    Examples of functional groups

    In the table above, we provided you with the names of molecules that are great examples of functional groups. Want to see these molecules in real life? Here they are.

    Alkanes, alkenes and alkynes

    Functional Groups alkane alkene alkyne example StudySmarter OriginalsFig. 1: Examples of alkanes, alkenes and alkyenes, with their functional groups highlighted. These molecules are all hydrocarbons.StudySmarter Originals

    This first picture gives you examples of the functional groups found in alkanes, alkenes, and alkynes. These three different families of organic molecules are all hydrocarbons, meaning that they contain just carbon and hydrogen atoms. They are characterised by their type of carbon-carbon bond. Alkanes have just C-C single bonds, whilst alkenes have C=C double bonds and alkynes have C≡C triple bonds.

    The C-C single bond, C=C double bond, and C≡C triple bond are all non-polar functional groups. This is because the only two elements involved in the group (carbon and hydrogen) both have very similar electronegativity values. Check out Polarity and Electronegativity for more.

    Alcohols and halogenoalkanes

    Functional Groups alcohol halogenoalkane example StudySmarter OriginalsFig. 2: Examples of the functional groups found in alcohols and halogenoalkanes: the hydroxyl group and a halogen atom respectively.StudySmarter Originals

    Alcohols contain the hydroxyl functional group. This consists of a hydrogen atom bonded to an oxygen atom, which is then in turn bonded to the rest of the molecule. We represent this group using -OH.

    The functional group in the halogenoalkane family is even simpler - it is just a halogen atom. We typically represent a halogen atom with X, giving halogenoalkanes the functional group -X.

    Both the hydroxyl group and halogen atoms are polar functional groups. Their polarity opens up the door to multiple different types of reactions, such as Nucleophilic Substitution Reactions. The hydroxyl group can also form hydrogen bonds and is easily converted into other functional groups, which you'll discover in Reaction of Alcohols.

    Aldehydes and ketones

    Functional Groups aldehyde ketone example StudySmarter OriginalsFig. 3: Examples of the functional group found in aldehydes and ketones: the carbonyl group.StudySmarter Originals

    Members of the aldehyde and ketone families all have variants of the same functional group: the carbonyl group, C=O. The carbonyl group itself consists of an oxygen atom joined to a carbon atom with a double bond. In the aldehyde functional group, the C=O is found at the end of a carbon chain. This means that the carbon atom is joined to one R group and one hydrogen atom. In the ketone functional group, the C=O is found in the middle of a carbon chain. This means that the carbon atom is joined to two R groups. Notice how the placement of the C=O carbonyl group is the only difference between the aldehyde propanal (above, left) and the ketone propanone (above, right). However, it gives aldehydes and ketones different characteristic reactions. Find out more in Reactions of Aldehydes and Ketones.

    Carboxylic acids, acyl chlorides and esters

    Functional Groups carboxylic acid acyl chloride ester example StudySmarter OriginalsFig. 4: Examples of the functional groups found in carboxylic acids, acyl chlorides, and esters: the carboxyl group, acyl chloride group, and ester link respectively.StudySmarter Originals

    Carboxylic acids contain the carboxyl functional group, -COOH. This group's name gives you some clues as to its structure. It is actually made up of two functional groups that we looked at earlier: the carbonyl group (C=O, found in aldehydes and ketones) and the hydroxyl group (-OH, found in alcohols). The combination of the two groups means that carboxylic acids react as typical acids by donating a proton in solution.

    Acyl chlorides and esters are both types of acid derivatives. Acid derivatives are close cousins of carboxylic acids, in which the hydroxyl group (-OH) has been replaced by another electronegative group. In acyl chlorides, the replacement group is a chlorine atom, which gives acyl chlorides the functional group -COCl (known as the acyl chloride group). In esters, the replacement group is an oxygen atom bonded to an additional R group. This gives esters the functional group -COO- (known as the ester link).

    Amines and amides

    Functional Groups amine amide example StudySmarter OriginalsFig. 5: Examples of the functional groups found in amines and amides: the amine group and the amide group respectively.StudySmarter Originals

    It's time to add an extra element to the mix: nitrogen. Our first example of a nitrogen-containing organic compound is an amine, which has a functional group with the same name. The amine group is formed from a nitrogen atom with a lone pair of electrons bonded to at least one organic R group. The presence of nitrogen's lone pair of electrons means that amines react as bases. Amine groups with one or more hydrogen atom can also form hydrogen bonds.

    For ease, we've shown an amine group bonded to one R group and two hydrogen atoms. This particular group is also known as an amino group and is specifically found in primary amines. But nitrogen atoms bonded to two or three R groups are still also considered amine groups and are found in secondary and tertiary amines respectively. You can see examples of all of these molecules in Amines.

    Amides contain nitrogen atoms too. They are characterised by the amide functional group, which in turn is made up of an amine group (-NH2) directly bonded to a carbonyl group (C=O). Like amines, they too form hydrogen bonds.

    Arenes, halogenoarenes and phenol

    Functional Groups arene halogenoarene phenol example StudySmarter OriginalsFig. 6: Examples of the functional groups found in arenes, halogenoarenes and phenols. All three families contain an aromatic arene ring. Halogenoarenes are made up of a phenol group bonded to a halogen atom whilst phenols contain a phenyl group bonded to the hydroxyl group.StudySmarter Originals

    The next three examples of organic families all share an aromatic benzene ring in common. Benzene rings are also known as the arene functional group and can be represented with the formula C6H6. However, it is much clearer to draw the arene group out, using a hexagon with a circle inside.

    Unlike the other hydrocarbons we've considered before (such as alkanes, alkenes, and alkynes), benzene rings contain a stable circle of delocalised pi electrons. This ring is strong and hard to break, which means that arenes and their derivatives tend to take part in substitution reactions instead of addition reactions. You'll learn more about this in Benzene Electrophilic Substitution.

    Arene derivatives include halogenoarenes and phenols. In arene derivatives, one or more of the arene ring's hydrogen atoms is swapped for a different group. We call an arene ring that is missing a hydrogen atom the phenyl group. For example, halogenoarenes exchange hydrogen atoms in the arene ring for halogen atoms, meaning that their functional group combines the phenyl group and a halogen atom. Phenols exchange hydrogen atoms for the hydroxyl group (-OH) found in alcohols, meaning that their functional group combines the phenyl group and the hydroxyl group.

    Functional group practice problems and examples

    Now that you have learned about different types of functional groups, it's time for you to put your knowledge to the test. Have a go at identifying the functional groups in various molecules with the help of our functional group practice questions.

    Identify the functional groups in the following molecule.

    Functional Groups practice question StudySmarterFig. 7: Can you spot the functional groups?StudySmarter Originals

    We can see that this molecule has three functional groups:

    1. A chlorine atom (-Cl) on the left.
    2. A C=C double bond in the middle.
    3. A carboxyl group (-COOH) on the right.

    We've highlighted them in the diagram below.

    Functional Groups practice question answers labelled diagram StudySmarterFig. 8: The functional groups in our unknown molecule, labelled.StudySmarter Originals

    Here's another practice question. This time, it uses skeletal formula.

    Identify the functional groups in the following organic molecule.

    Functional Groups practice question StudySmarterFig. 9: Can you spot the functional groups?StudySmarter Originals

    This molecule is a little trickier. We need to pay special attention to the C=O bonds in order to figure out which functional group they belong to. Like in the first example, this molecule has three functional groups in total.

    1. The C=O bond on the left is joined to an amine group (-NH2). The two combine to form an amide group (-CONH2).
    2. The C=O bond in the middle of the molecule is joined to two R groups. It is therefore a ketone carbonyl group (-CO-).
    3. There is also a hydroxyl group (-OH) on the right.

    Once again, we've labeled the functional groups in the diagram down below.

    Functional Groups practice question answers labelled diagram StudySmarterFig. 10: The functional groups in our second unknown molecule, labeled.StudySmarter Originals

    If you've read the article IUPAC Nomenclature, you might feel like trying to name the two unfamiliar organic structures in our practice questions.

    The first molecule is 3-chloroprop-2-enoic acid.

    The second molecule is 3-hydroxy-2-oxopropanamide.

    Don't worry if you don't get their names quite right - these are complicated species!

    Here's a different sort of question.

    Draw a molecule with four carbon atoms, an alkyne group, an acyl chloride group, and a carboxyl group.

    If you think about this problem logically, there is only one possible molecule. The acyl chloride group and carboxyl group must go on either end of the carbon chain. The carbon atoms in these groups already have three covalent bonds and so they can't be involved in the alkyne group, which is characterised by its C≡C triple bond. This means that the alkyne group must go in the middle of the molecule. We've drawn the final structure down below and labeled the function groups.

    Functional Groups practice question answers labelled diagram StudySmarterFig. 11: Our third unknown molecule. Did you draw the correct functional groups?StudySmarter Originals

    Functional Groups - Key takeaways

    • Functional groups are specific groups of atoms that are responsible for a molecule's characteristic reactions and properties.
    • Molecules in the same homologous series (also known as the same family) have the same functional group. This means that they react in similar ways.
    • Examples of functional groups include:
      • C-C single, C=C double, and C≡C triple bonds.
      • Halogen atoms (-X).
      • The hydroxyl group (-OH).
      • The carbonyl group (C=O), found in aldehydes (-CHO) and ketones (-CO-).
      • The carboxyl group (-COOH) and its derivatives such as the acyl chloride group (-COCl) and ester link (-COO-).
      • The amine group (-NH2) and amide group (-CONH2).
      • The arene group (C6H6), found in arenes, halogenoarenes, and phenols.
    Frequently Asked Questions about Functional Groups

    What is a functional group?

    Functional groups are specific groups of atoms that are responsible for a molecule's characteristic reactions and properties.

    What are the main functional groups?

    Examples of the main functional groups include:

    • C-C single, C=C double, and C≡C triple bonds.
    • The hydroxyl group (-OH).
    • The carbonyl group (C=O).
    • The carboxyl group (-COOH).
    • The amine group (-NH2).

    How do you identify functional groups?

    To identify functional groups, look at a molecule's structure and identify any parts that aren't C-C single bonds. Consider each unfamiliar part and compare it to a functional group table to identify the functional group present. You should also consider the various atoms and bonds surrounding each unfamiliar part to see if it is part of a larger functional group. For example, at first glance, the structure -CONH2 might look like an amine group bonded to a carbonyl group, but together these two functional groups make up the amide group.

    Why are functional groups important?

    Functional groups are important because they determine a molecule's characteristic reactions and properties. Molecules with the same functional group all take part in similar reactions. Because of this, we can confidently predict how an unfamiliar molecule reacts just by looking at its structure.

    What are the properties of functional groups?

    Functional groups have different properties depending on their structure and arrangement of atoms. For example, some functional groups are non-polar whilst others are polar. Other functional groups might form hydrogen bonds or dissolve in non-polar solvents.

    Test your knowledge with multiple choice flashcards

    True or false? All molecules in a homologous series contain the same functional group? 

    True or false? All molecules in a homologous series have the same length carbon chain.

    Which of the following statements are true about the hydroxyl group?

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