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25.1: Hydrocarbons – The Backbone of Organic Chemistry

Created by: CK-12

Lesson Objectives

  • Describe the bonding characteristics of carbon.
  • Differentiate between saturated and unsaturated hydrocarbons.
  • Draw and name structures for simple hydrocarbons.

Lesson Vocabulary

  • organic chemistry: A field of chemistry which studies the structure and reactivity and nearly all carbon-containing compounds.
  • hydrocarbon: Molecules that contain only carbon and hydrogen atoms.
  • alkane: Hydrocarbons in which all carbons are connected by single bonds.
  • alkene: A compound in which a C=C double bond is present.
  • alkyne: A compound in which a C≡C triple bond is present.
  • saturated: Hydrocarbons which contain no multiple bonds.
  • unsaturated: Hydrocarbons which contain at least one double or triple bond.

Check Your Understanding

Recalling Prior Knowledge

  • How many covalent bonds does each carbon atom in a molecule usually make?
  • How can the hybridization of a given carbon atom be determined?

Introduction

Before the 19th century, scientists had believed that the chemical processes occurring in living systems were fundamentally different from those that could be observed in a test tube. They classified chemistry into two categories: organic and inorganic. Organic processes were thought to take place only in living systems, while inorganic processes occurred in material that was not living. A "vital force" was believed to be necessary for organic reactions to occur. This way of thinking was challenged in 1828 by the German chemist Friedrich Wöhler when he synthesized an organic compound (urea, found in urine) from an inorganic precursor (ammonium cyanate):

Reaction of ammonium cyanate to form urea.

Since then, the distinction between organic and inorganic compounds and reactions has blurred. Currently, the field of organic chemistry studies the structure and reactivity of nearly all carbon-containing compounds. Over twenty million organic compounds are known, ranging from very simple molecules to complex proteins.

Bonding and Hybridization in Carbon

Let’s briefly review the basics of covalent bonding as they pertain to carbon. Carbon has four valence electrons, which have a 2s22p2 configuration in isolated carbon atoms. These four electrons allow carbon to form four covalent bonds, which can mean four single bonds or some combination of single, double, and triple bonds.

A carbon atom that has formed single bonds to four different atoms has an sp3 hybridization. The angles between these bonds are equal to 109.5°.

Recall that a double bond consists of one sigma bond and one pi bond. In order for a double bond to be formed, each participating carbon atom must have at least one unhybridized p orbital. In a carbon-carbon double bond where both carbons are bonded to two additional atoms, each carbon is sp2 hybridized. The double bond includes a sigma bond between a hybrid orbital from each carbon and a pi bond between the leftover p orbital from each carbon. The angles between any two bonds for an sp2 hybridized carbon are approximately 120°.

Hybridization of the valence orbitals in a carbon atom to make a set of three sp2 orbitals, with one p orbital left over.

A triple bond (-C≡C-) requires each of the carbon atoms to be sp hybridized. One hybrid orbital and two p orbitals from each atom are involved in forming the one sigma and two pi bonds that make up a triple bond. Each carbon atom is also bonded to one other atom via the other hybrid orbital. The angle between these two bonds for an sp hybridized carbon is 180°.

Hybridization of the valence orbitals in a carbon atom to make a set of two sp orbitals, with two p orbitals left over.

Hydrocarbon Structure and Naming

Hydrocarbons are molecules that contain only carbon and hydrogen atoms. Because each carbon atom can form covalent bonds with up to four other atoms, very large and complex molecules can be formed just from these two elements. Hydrocarbons in which all carbons are connected by single bonds are known as alkanes. If a C=C double bond is present, the compound is now an alkene. A triple bond between two carbons (C≡C) makes the compound an alkyne. Hydrocarbons can also be broadly classified as either saturated, which means they contain no multiple bonds, or unsaturated, which means they contain at least one double or triple bond.

The simplest alkanes are linear chains of carbon atoms, in which no carbon is bonded to more than two other carbon atoms. Branched alkanes are also possible, greatly increasing the complexity of possible structures that can be formed from a given set of carbon and hydrogen atoms. The first six linear alkanes are listed in the Table below:

Structure Name
CH4 methane
CH3CH3 ethane
CH3CH2CH3 propane
CH3CH2CH2CH3 butane
CH3CH2CH2CH2CH3 pentane
CH3CH2CH2CH2CH2CH3 hexane

Starting with pentane, linear alkanes are named by adding "-ane" to the Latin prefix corresponding to the number of carbon atoms in the chain.

Since organic chemistry is essentially carbon chemistry, it is important to understand the structure of the hydrocarbon chain. Although alkanes are relatively unreactive, they provide the backbone for more reactive structures known as functional groups, which we will discuss in the following lesson. Most organic reactions will alter only specific functional groups, while the hydrocarbon backbone is generally left intact.

Drawing Organic Structures

We can indicate hydrocarbon structures in several ways. The entire structure of hexane is shown below using the usual rules for drawing Lewis structures. Each atom is indicated with the symbol of its element, and each single covalent bond is represented with a line.

Hexane structure, with all atoms shown.

However, this type of structure is time consuming to draw and can become very cluttered. Because carbons and hydrogen atoms are so prevalent in organic molecules, a chemical shorthand was developed so that not all atoms need to be explicitly drawn. Below is another way to draw the hexane molecule:

Skeleton structure for hexane.

In the above structure, the two ends of the chain and each of the intervening corners represents a carbon atom. The six carbon atoms are connected in a linear chain by single bonds. Unless indicated otherwise, we also assume that each carbon makes four total bonds, and any bonds that are not explicitly drawn are connected to hydrogen atoms. The internal carbon atoms above each make single bonds to two other carbons, leaving two bonds not shown. Thus, each of these carbon atoms is connected to two hydrogens. The carbons on the ends of the chain only have one covalent bond drawn in, so they must each be bonded to three hydrogen atoms. Compare these two representations of hexane, keeping in mind that both are conveying the same information.

Locating Functional Groups

Most organic compounds are not simple hydrocarbons; they have functional groups that provide additional reactivity pathways. To indicate the location of a functional group within the name of an organic molecule, the hydrocarbon backbone is generally numbered. For example, the hexane molecule (see above) could serve as a parent chain. It has six carbons in it, which can be numbered C-1, C-2, and so on. As long as there is nothing else attached to the chain, it does not matter where we start counting. There is no way to designate which carbon is C-1 and which carbon is C-6. However, once a substituent is added to the chain, we can then indicate a start and an end to the molecule.

Now, let's introduce a functional group by replacing one of the hydrogen atoms in hexane with a chlorine atom:

2-Chlorohexane

To indicate that this molecule has a chlorine atom attached to the hydrocarbon backbone, we could name this compound chlorohexane. However, that name would not be enough information to uniquely identify this molecule, since the chlorine could be attached to any of the carbon atoms. To indicate the location of this substituent, we number the chain, starting with the end that will place the functional group on the carbon atom with the lowest number. Depending on which end is C-1, the above compound could be called either 2-chlorohexane or 5-chlorohexane. According to our rule about giving functional groups the lowest possible numbers, this molecule would be called 2-chlorohexane.

Other alkanes with a single halogen atom can be named using a similar strategy, except chloro would be replaced by fluoro, bromo, or iodo, depending on the identity of the halogen.

The location of double and triple bonds must also be indicated with numbers. For example, consider the following two molecules: CH3CH2CH=CHCH3 and CH2=CHCH2CH2CH3. Both of these have one double bond. A simple 5-carbon alkane (no double bonds) would be called pentane, so adding in one double bond changes the name of the structure to pentene, since it is an alkene. However, the location of the double bond affects the physical and chemical properties of the compound.

In order to distinguish between the two molecules above, we again number the carbon chain, starting from the end that will give the functional group (the alkene) the lowest number. For CH3CH2CH=CHCH3, we would start counting on the right end. The double bond is between carbon atoms 2 and 3, so this molecule would be named pent-2-ene, where the lower of the two numbers is used. For CH2=CHCH2CH2CH3, we would start counting on the left end. The double bond is between carbon atoms 1 and 2, so this molecule would be named pent-1-ene.

Triple bonds can be identified in a similar way, except that the suffix -yne is used instead of -ene (to indicate that we are dealing with an alkyne instead of an alkene).

Cyclic Hydrocarbons

Many organic compounds are cyclic in structure. The compound cyclohexane involves a ring of six carbon atoms, each of which is also bonded to two hydrogen atoms. Figure below shows a few different representations of the cyclohexane molecule.

Ways of representing the structure of cyclohexane.

The structure on the right gives the complete picture, where all atoms are explicitly drawn. The middle structure shows a flat representation of the molecule based on the standard shorthand rules for drawing organic structures. The folded structure on the left highlights an important point about organic chemistry – the three-dimensional structure of a molecule is not always portrayed accurately by flat drawings. The true structure of the cyclohexane molecule has a puckered shape that looks more like the structure on the left than the flat hexagon in the center. The preferred three-dimensional conformations of organic molecules often play an important role in how the molecule reacts. The following structures illustrate some of the interesting and complex shapes organic molecules can take on:

Complex organic structures.

Aromatic hydrocarbons are a special subset of cyclic hydrocarbons. Although many "aromatic" compounds have distinctive odors, this word is used very differently in organic chemistry than in everyday life. The benzene ring is the foundational structure for most aromatic compounds:

Ways to represent benzene ring.

The illustrations above give different perspectives on the actual structure of the molecule. The left-hand illustration shows the six hydrogen atoms attached to the six carbons and indicates that there are three double bonds in the ring, while the next structure shows this symbolically. A more realistic picture is given by the next two models. The circle shows the reality of the bonding. The three pi bonds in the ring overlap one another and form a cloud of electrons above and below the plane of the ring. Benzene and its derivatives do not undergo the same reactions as most carbon-carbon double bonds, due to the special stability that is inherent in this type of interactive pi bonding.

Lesson Summary

  • Hydrocarbons contain only carbon and hydrogen.
  • Alkanes contain only carbon-carbon single bonds.
  • Alkenes contain one or more carbon-carbon double bonds.
  • Alkynes contain one or more carbon-carbon triple bonds.
  • Chemists use a shorthand for drawing organic structures that focuses on functional groups and simplifies the drawing of the hydrocarbon backbone.
  • Many hydrocarbons are cyclic and adopt specific three-dimensional structures that influence their physical and chemical properties.
  • Aromatic compounds are cyclic and have a cloud of pi electrons above and below the plane of the ring.

Lesson Review Questions

  1. What is a hydrocarbon?
  2. Classify the following hydrocarbons as saturated or unsaturated, and identify each as an alkane, alkene, or alkyne:
    1. CH3CH2CH2CH3
    2. CH3CH=CHCH2CH3
    3. CH3C≡CH
  3. Name each of the compounds in the previous problem.

Further Reading/Supplementary Links

Points to Consider

  • Is there a systematic way to classify organic compounds?

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Date Created:

Sep 09, 2013

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Aug 28, 2014
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