Introduction to Organic Chemistry
How one element builds fuels, plastics, flavors and you
Natural gas, banana flavor, vinegar, a plastic bag and the DNA in your cells are all built around carbon. Organic chemistry is the chemistry of carbon compounds. Carbonates, cyanides and simple oxides such as CO and CO2 are usually left out,[1] and that still leaves a vast number of compounds. In this unit you will see why carbon is special, name the hydrocarbons, learn to spot functional groups, meet isomers and polymers, and connect it all to the molecules of life.
This unit builds on covalent bonds from Chemical Bonding. Try the molecule builder and the functional-group spotter, then take the quiz.
Why Carbon Is Special
A carbon atom has four valence electrons, so it forms four covalent bonds. Carbon can make up to four strong bonds to other carbon atoms, building chains and rings of many sizes, shapes and complexities.[2] Two carbons can share one, two or three pairs of electrons: a single, double or triple bond.
Few other elements can link to themselves so many times so strongly. That is why chemists have a separate branch of chemistry for carbon compounds.
Hydrocarbons: Alkanes, Alkenes and Alkynes
Hydrocarbons contain only carbon and hydrogen. We burn them every day: methane, CH4, is the main component of natural gas.[2]
- Alkanes (saturated hydrocarbons) contain only single bonds between carbon atoms.[2] Their names end in -ane. Chains without rings follow the formula CnH2n+2.[2]
- Alkenes contain one or more carbon–carbon double bonds and end in -ene, like ethene (ethylene).[2] One double bond means two fewer hydrogens than the alkane: CnH2n.
- Alkynes contain one or more triple bonds and end in -yne, like ethyne (acetylene).[2] One triple bond means four fewer hydrogens: CnH2n−2.
The prefix tells you the number of carbons in the main chain:
| Carbons | Prefix | Alkane | Formula | Boiling point (°C) |
|---|---|---|---|---|
| 1 | meth- | methane | CH4 | −161.5 |
| 2 | eth- | ethane | C2H6 | −88.6 |
| 3 | prop- | propane | C3H8 | −42.1 |
| 4 | but- | butane | C4H10 | −0.5 |
| 5 | pent- | pentane | C5H12 | 36.1 |
| 6 | hex- | hexane | C6H14 | 68.7 |
| 7 | hept- | heptane | C7H16 | 98.4 |
| 8 | oct- | octane | C8H18 | 125.7 |
| 9 | non- | nonane | C9H20 | 150.8 |
| 10 | dec- | decane | C10H22 | 174.0 |
Boiling points are for the straight-chain (normal) isomer of each alkane.[2]
Longer chains boil at higher temperatures because the molecules have more surface to attract one another. The first four alkanes are gases at room temperature; pentane through decane are liquids.
Worked example: what is the formula of heptane, and of 1-heptene?
- Hept- means 7 carbons. Alkane: C7H2×7+2 = C7H16.
- 1-Heptene has one double bond, so two fewer H: C7H14. The 1 says the double bond starts at the first carbon.
Three Ways to Write a Molecule
A structural formula draws every atom and bond. A condensed formula writes the atoms bonded to each carbon in order, such as CH3CH2CH2CH3. A skeletal formula is the shorthand chemists use most: carbon atoms are not written as C but sit at each end and bend of a line, and hydrogens attached to carbon are left out.[2] The zigzag is not just decoration: bond angles in alkane chains are close to 109.5°.[2]
Worked example: how many hydrogens are hidden in a skeletal formula?
Every carbon makes four bonds. Count the lines at each corner or end and add hydrogens to make four. In butane each end carbon has 1 line, so 3 H; each middle carbon has 2 lines, so 2 H. Total 3 + 2 + 2 + 3 = 10, matching C4H10.
Molecule Builder
Pick the number of carbons and the bond between the first two. The builder names the molecule, writes its formula and draws it.
Butane, C4H10, an alkane. Butane boils at −0.5 °C. It has 2 structural isomers with this formula.
Methane (1 carbon) has no carbon–carbon bond, so it can only be an alkane. Molecules with one or two carbons are drawn with every atom, because a skeletal formula of a single carbon is just a dot.
Boiling points from OpenStax Table 20.1.[2]
Functional Groups
A functional group is a particular arrangement of atoms that gives a molecule its typical reactions. Swap one hydrogen on ethane for –OH and you have ethanol, an alcohol;[3] the functional group largely decides how the molecule reacts. Many groups contain the carbonyl group, C=O: in an aldehyde the carbonyl carbon is bonded to at least one hydrogen, and in a ketone it is bonded to two carbons.[4]
| Group | What to look for | Everyday examples |
|---|---|---|
| Alcohol | –OH | ethanol; ethylene glycol in antifreeze[3] |
| Ether | C–O–C | diethyl ether, a highly flammable solvent[3] |
| Aldehyde | C=O with at least one H on the carbon | formaldehyde[4] |
| Ketone | C=O between two carbons | acetone, the simplest ketone[4] |
| Carboxylic acid | –COOH | acetic acid, 3–6% of vinegar[4] |
| Ester | –COO– between two carbons | the smell of ripe bananas and many other fruits; fats and oils[4] |
| Amine | N with three bonds and a lone pair | amino acids, hormones, neurotransmitters, DNA[5] |
Amines contain nitrogen with a lone pair and three bonds.[5] Aldehydes take the suffix -al (methanal is formaldehyde) and ketones take -one (propanone is acetone).
Functional-Group Spotter
Each boxed part of these molecules is a functional group. Name it.
Isomers
Isomers have the same molecular formula but different structures. Butane and isobutane (2-methylpropane) are both C4H10: one is a straight chain and the other is branched.[2] The number of possible structures grows fast. Decane, C10H22, has 75 structural isomers.[2]
Cis and trans. A double bond is rigid and cannot twist, so 2-butene comes in two forms: with both methyl groups on the same side of the double bond (cis) or on opposite sides (trans).[2]
Mirror images. A carbon bonded to four different groups can exist in two forms that are mirror images, related like your right and left hands. These enantiomers cannot be laid exactly on top of each other.[12]
Polymers
Polymers (Greek poly, “many”, and mer, “parts”) are large molecules made of repeating units called monomers.[2]
Addition polymers form when monomers with double bonds join end to end and nothing is left over. Ethylene, CH2=CH2, links up into long chains of –CH2– units: polyethylene.[2]
Condensation polymers form when each new link releases a small molecule, usually water. Reactions in which two molecules combine and release water are called condensation reactions.[5] In the 1930s Wallace Carothers's team at DuPont made polyesters and then polyamides, linking molecules through amines instead of alcohols;[9] one polyamide became nylon, in production from 1939.[9]
Many plastics are labeled with a recycling code that tells sorting centers which plastic they are.[2] Check the code against what your local program accepts.
Molecules of Life
Living things are organic chemistry in action. Biologists group the big molecules of life into four classes: carbohydrates, lipids, proteins and nucleic acids.[6] Most are polymers built by dehydration synthesis, “to put together while losing water”, a condensation reaction; hydrolysis adds water to break them apart again.[6]
- Carbohydrates are made of carbon, hydrogen and oxygen.[3] Glucose monomers build starch, glycogen and cellulose.[6]
- Lipids: fats and oils are esters of glycerine with large carboxylic acids.[4]
- Proteins are polymers of amino acids, molecules containing both an amine and a carboxylic acid group.[5]
- Nucleic acids: DNA is a polymer built from four kinds of nucleotides.[5]
Learn what these molecules do inside a living cell in The Cell.
Safety With Flammable Solvents
- Many organic liquids catch fire easily. Diethyl ether is a volatile, highly flammable liquid,[3] and acetone is classed as a flammable liquid.[14]
- Keep organic solvents away from flames, hot plates and sparks. Their vapors can spread to a flame some distance away.
- Use only small amounts, keep the bottle capped, and work in a fume hood if your teacher says to.
- Take off clothing that gets soaked with a flammable solvent.[14] Never pour solvents down the drain; use the waste container your teacher provides.
How Do We Know?
Wöhler makes urea (1828)
Chemists once believed in vitalism: organic compounds could only be made by a vital force present in living things.[1] In 1828 Friedrich Wöhler reported making urea, a compound found in body fluids, from nonliving materials.[1] He had been trying to make ammonium cyanate.[7] He wrote to his mentor Berzelius: “I can no longer, so to speak, hold my chemical water and must tell you that I can make urea without needing a kidney, whether of man or dog; the ammonium salt of cyanic acid is urea.”[7]
Kekulé, carbon chains and the benzene ring (1857 to the 1860s)
August Kekulé proposed in 1857 that carbon is tetravalent (forms four bonds). In 1858 he and Archibald Scott Couper each realized that carbon atoms can link directly to one another in chains.[8] By Kekulé's own later account, while he dozed by the fire one evening, probably in 1862, he pictured a chain of carbon atoms that took its tail into its mouth like a snake. That image gave him the idea of a ring for benzene.[8]
Seeing the bonds (2009)
Skeletal formulas were once only a model. In 2009 Leo Gross and colleagues at IBM Research in Zurich used a noncontact atomic force microscope, its tip capped with a single carbon monoxide molecule, to image a pentacene molecule, C22H14.[10][11] The five hexagonal carbon rings showed up clearly, much as chemists had been drawing them.[10]
Test Yourself
Ten questions. The molecule builder is allowed!
The Carbon Challenge
Think About It
Questions to puzzle over in class or at home.
- Propane and butane are both sold as fuels in pressurized tanks. Using their boiling points, explain why both turn back into gas as soon as they leave the tank.
- Why can't methane have a double bond? What is the smallest alkene?
- Butane and isobutane have the same formula. Why might their boiling points still differ?
- Your body breaks starch into glucose. Is that dehydration synthesis or hydrolysis? What does it need?
- Before 1828 many chemists thought organic compounds needed a living thing to make them. What kind of experiment would convince you otherwise?
Sources
- OpenStax, Chemistry 2e, “Chapter 20 Introduction (Organic Chemistry).” Quotes: “The widespread belief called vitalism held that organic compounds were formed by a vital force present only in living organisms.” “The German chemist Friedrich Wohler was one of the early chemists to refute this aspect of vitalism, when, in 1828, he reported the synthesis of urea, a component of many body fluids, from nonliving materials.” “Some carbon-containing compounds are not classified as organic, for example, carbonates and cyanides, and simple oxides, such as CO and CO2.” https://openstax.org/books/chemistry-2e/pages/20-introduction
- OpenStax, Chemistry 2e, “20.1 Hydrocarbons.” Quotes: “The existence of so many organic molecules is a consequence of the ability of carbon atoms to form up to four strong bonds to other carbon atoms, resulting in chains and rings of many different sizes, shapes, and complexities.” “Alkanes, or saturated hydrocarbons, contain only single covalent bonds between carbon atoms.” “noncyclic alkanes all have a formula of CnH2n+2”. “the bond angles in carbon chains are close to 109.5°, giving such chains in an alkane a zigzag shape”. Skeletal structures: “carbon atoms are not symbolized with a C, but represented by each end of a line or bend in a line. Hydrogen atoms are not drawn if they are attached to a carbon.” “Unsaturated hydrocarbon molecules that contain one or more double bonds are called alkenes”. “Hydrocarbon molecules with one or more triple bonds are called alkynes”. “Molecules of 1-butene and 2-butene are structural isomers”. On 2-butene: “one with both methyl groups on the same side of the double bond and one with the methyl groups on opposite sides”, “cis-isomer”, “trans-isomer”. “Polymers (from Greek words poly meaning “many” and mer meaning “parts”) are large molecules made up of repeating units, referred to as monomers.” “methane, CH4, is the principal component of natural gas.” “Many areas of the country have recycling programs that focus on one or more of the commodity plastics that have been assigned a recycling code”. Table 20.1 boiling points (°C), with the note “physical properties for C4H10 and heavier are those of the normal isomer”: methane –161.5, ethane –88.6, propane –42.1, butane –0.5, pentane 36.1, hexane 68.7, heptane 98.4, octane 125.7, nonane 150.8, decane 174.0; number of structural isomers 1, 1, 1, 2, 3, 5, 9, 18, 35, 75. https://openstax.org/books/chemistry-2e/pages/20-1-hydrocarbons
- OpenStax, Chemistry 2e, “20.2 Alcohols and Ethers.” Quotes: “Alcohols are derivatives of hydrocarbons in which an –OH group has replaced a hydrogen atom.” “1,2-ethanediol (ethylene glycol, used in antifreeze)”. “Ethers are compounds that contain the functional group –O–.” “Diethyl ether, the most widely used compound of this class, is a colorless, volatile liquid that is highly flammable.” “Carbohydrates are large biomolecules made up of carbon, hydrogen, and oxygen.” https://openstax.org/books/chemistry-2e/pages/20-2-alcohols-and-ethers
- OpenStax, Chemistry 2e, “20.3 Aldehydes, Ketones, Carboxylic Acids, and Esters.” Quotes: “In an aldehyde, the carbonyl group is bonded to at least one hydrogen atom. In a ketone, the carbonyl group is bonded to two carbon atoms.” “Dimethyl ketone, CH3COCH3, commonly called acetone, is the simplest ketone.” “The odor of vinegar is caused by the presence of acetic acid”. “Acetic acid, CH3CO2H, constitutes 3–6% vinegar”. “The odor of ripe bananas and many other fruits is due to the presence of esters”. Fats and oils “are esters of the trihydroxyl alcohol glycerine”. https://openstax.org/books/chemistry-2e/pages/20-3-aldehydes-ketones-carboxylic-acids-and-esters
- OpenStax, Chemistry 2e, “20.4 Amines and Amides.” Quotes: “The nitrogen atom in an amine has a lone pair of electrons and three bonds to other atoms”. Amines “are also found in many molecules essential to life, such as amino acids, hormones, neurotransmitters, and DNA.” “reactions that result in the production of water when two other molecules combine are referred to as condensation reactions”. “amino acids (molecules containing both amine and carboxylic acid substituents) link together in a polymer to form proteins.” “The four molecules that make up DNA are called nucleotides.” https://openstax.org/books/chemistry-2e/pages/20-4-amines-and-amides
- OpenStax, Biology 2e, “3.1 Synthesis of Biological Macromolecules.” Quotes: “There are four major biological macromolecule classes (carbohydrates, lipids, proteins, and nucleic acids).” “In doing so, monomers release water molecules as byproducts. This type of reaction is dehydration synthesis, which means “to put together while losing water.”” “glucose monomers are the constituents of starch, glycogen, and cellulose.” “Polymers break down into monomers during hydrolysis.” https://openstax.org/books/biology-2e/pages/3-1-synthesis-of-biological-macromolecules
- Science History Institute, “Justus von Liebig and Friedrich Wöhler.” Quotes: “Perhaps the most famous creation of an isomeric compound was Wöhler’s accidental synthesis of urea in 1828, when he was attempting to prepare ammonium cyanate”. From “his often-quoted letter to Berzelius: “I can no longer, so to speak, hold my chemical water and must tell you that I can make urea without needing a kidney, whether of man or dog; the ammonium salt of cyanic acid is urea.”” https://www.sciencehistory.org/education/scientific-biographies/justus-von-liebig-and-friedrich-wohler/
- Science History Institute, “August Kekulé and Archibald Scott Couper.” Quotes: “In 1858 August Kekulé … and Archibald Scott Couper … independently recognized that carbon atoms can link directly to one another to form carbon chains.” “Kekulé’s theory, proposed in 1857, that carbon is tetravalent”. “Again according to his later reminiscences, one evening in his residence in Ghent (probably in 1862) … he saw one that, snakelike, took its tail into its mouth, which gave him the idea for the ring form of the benzene molecule.” https://www.sciencehistory.org/education/scientific-biographies/august-kekule-and-archibald-scott-couper/
- Science History Institute, “Wallace Hume Carothers.” Quotes: “Through his inventions of nylon and neoprene in 1930, Carothers helped demonstrate the macromolecular theory of polymers”. “he and his team used amines rather than glycols to produce polyamides rather than polyesters.” “Nylon went into production in 1939”. https://www.sciencehistory.org/education/scientific-biographies/wallace-hume-carothers/
- IBM Research – Zurich, “IBM Scientists First to Image the ‘Anatomy’ of a Molecule” (news release, August 28, 2009; archived copy). Quotes: “In the present experiments, the molecule investigated was pentacene. Pentacene is an oblong organic molecule consisting of 22 carbon atoms and 14 hydrogen atoms measuring 1.4 nanometers in length.” “In the experimental image, the hexagonal shapes of the five carbon rings as well as the carbon atoms in the molecule are clearly resolved.” A tip “terminated with a carbon monoxide (CO) molecule yielded the optimum contrast”. https://web.archive.org/web/20130512102118/http://www.zurich.ibm.com/news/09/pentacene.html
- L. Gross, F. Mohn, N. Moll, P. Liljeroth, G. Meyer, “The chemical structure of a molecule resolved by atomic force microscopy,” Science 325 (5944), 2009 (PubMed record). Quote from the abstract: “We demonstrate imaging of molecules with unprecedented atomic resolution by probing the short-range chemical forces with use of noncontact atomic force microscopy.” https://pubmed.ncbi.nlm.nih.gov/19713523/
- OpenStax, Organic Chemistry, “5.1 Enantiomers and the Tetrahedral Carbon.” Quote: “Enantiomers are related to each other as a right hand is related to a left hand and result whenever a tetrahedral carbon is bonded to four different substituents (one need not be H).” https://openstax.org/books/organic-chemistry/pages/5-1-enantiomers-and-the-tetrahedral-carbon
- PubChem, “Aspirin” (CID 2244), description from ChEBI. Quote: “Acetylsalicylic acid is a member of the class of benzoic acids that is salicylic acid in which the hydrogen that is attached to the phenolic hydroxy group has been replaced by an acetoxy group.” https://pubchem.ncbi.nlm.nih.gov/compound/2244
- NIOSH Pocket Guide to Chemical Hazards, “Acetone.” Quotes: “Class IB Flammable Liquid: Fl.P. below 73°F and BP at or above 100°F.” Personal protection: “Remove: When wet (flammable)”. https://www.cdc.gov/niosh/npg/npgd0004.html