The lobes of the p orbitals prevent the atoms sharing pi bonds from rotating as shown in the diagram below. The pi bonds in double and triple bonds prevent these bonds from rotating.
This rigidity has an effect on the physical structure of compounds and can influence chemical reactivity. For now, we want to build the habit of looking at static drawings and diagrams of organic compounds and visualizing their dynamic nature. The ionic bond is fairly intuitive.
On the other hand, the covalent bond is supposed to result from the sharing of 2 electrons between nuclei. But why should a pair of electrons bind together? The modern covalent bond is conceived to be a region of high electron density between 2 positively charged atomic nuclei.
The atomic nuclei are attracted to the region of electron density. If the nuclei get too close, the positive nuclei repel each other electrostatically, if they get too far from each other, their mutual attraction diminishes. The sweet spot, the equilibrium distance, where mutual attraction is maximized, and internuclear repulsion is minimized is by definition the equilibrium bond length.
The 2 electrons assumed to be derive from 1 electron from each atom combine in a bonding orbital, that whose equilibrium distance defines the bond length. Double bonds have shorter distances than single bonds, and triple bonds are shorter than double bonds. Boundless vets and curates high-quality, openly licensed content from around the Internet.
This particular resource used the following sources:. Skip to main content. Basic Concepts of Chemical Bonding. Search for:. Double and Triple Covalent Bonds. Learning Objective Describe the types of orbital overlap that occur in single, double, and triple bonds. Key Points Double and triple covalent bonds are stronger than single covalent bonds and they are characterized by the sharing of four or six electrons between atoms, respectively.
Just because you see dashed and solid wedges in a structure, do not automatically assume that you are looking at a stereocenter. Other elements in addition to carbon can be stereocenters. The phosphorus center of phosphate ion and organic phosphate esters, for example, is tetrahedral, and thus is potentially a stereocenter.
Having trouble visualizing chirality and enantiomers? It may be helpful to watch this. The thalidomide that was used in the s to treat depression and morning sickness was sold as a mixture of both the R and the S enantiomer — this is referred to as a racemic mixture.
The problem with racemic thalidomide, as we learned above, is that only the R enantiomer is an effective medicine, while the S enantiomer causes mutations in the developing fetus. How does such a seemingly trivial structural variation lead to such a dramatic and in this case, tragic difference in biological activity?
Virtually all drugs work by interacting in some way with important proteins in our cells: they may bind to pain receptor proteins to block the transmission of pain signals, for instance, or clog up the active site of an enzyme that is involved in the synthesis of cholesterol.
Instead, it seems that S -thalidomide interacts somehow with a protein involved in the development of a growing fetus, eventually causing the observed birth defects.
Source: www. The over-the-counter painkiller ibuprofen is currently sold as a racemic mixture, but only the S enantiomer is effective.
Fortunately, the R enantiomer does not produce any dangerous side effects, although its presence does seem to increase the amount of time that it takes for S -ibuprofen to take effect. You can, with the assistance your instructor, directly experience the biological importance of stereoisomerism.
Carvone is a chiral, plant-derived molecule that smells like spearmint in the R form and caraway a spice in the S form. The two enantiomers interact differently with smell receptor proteins in your nose, generating the transmission of different chemical signals to the olfactory center of your brain. The number of known organic compounds is a quite large. In fact, there are many times more organic compounds known than all the other inorganic compounds discovered so far, about 7 million organic compounds in total.
Fortunately, organic chemicals consist of a relatively few similar parts, combined in different ways, that allow us to predict how a compound we have never seen before may react, by comparing how other molecules containing the same types of parts are known to react.
These parts of organic molecules are called functional groups and are made up from specific bonding patterns with the atoms most commonly found in organic molecules C, H, O, N, S, and P.
The identification of functional groups and the ability to predict reactivity based on functional group properties is one of the cornerstones of organic chemistry.
Functional groups are specific atoms, ions, or groups of atoms having consistent properties. A functional group makes up part of a larger molecule. For example, -OH, the hydroxyl group that characterizes alcohols, is an oxygen with a hydrogen attached.
It could be found on any number of different molecules. Just as elements have distinctive properties, functional groups have characteristic chemistries. An -OH functional group on one molecule will tend to react similarly, although perhaps not identically, to an -OH on another molecule.
Organic reactions usually take place at the functional group, so learning about the reactivities of functional groups will prepare you to understand many other things about organic chemistry.
Functional groups are structural units within organic compounds that are defined by specific bonding arrangements between specific atoms. The structure of capsaicin, the fiery compound found in hot peppers, incorporates several functional groups, labeled in the figure below and explained throughout this section. As we progress in our study of organic chemistry, it will become extremely important to be able to quickly recognize the most common functional groups, because they are the key structural elements that define how organic molecules react.
For now, we will only worry about drawing and recognizing each functional group, as depicted by Lewis and line structures. Much of the remainder of your study of organic chemistry will be taken up with learning about how the different functional groups behave in organic reactions.
Below is a brief introduction to the major organic functional groups. Methane, CH 4 , is the natural gas you may burn in your furnace. Octane, C 8 H 18 , is a component of gasoline.
Alkenes sometimes called olefins have carbon-carbon double bonds, and alkynes have carbon-carbon triple bonds. Ethene, the simplest alkene example, is a gas that serves as a cellular signal in fruits to stimulate ripening. If you want bananas to ripen quickly, put them in a paper bag along with an apple — the apple emits ethene gas also called ethylene , setting off the ripening process in the bananas.
Ethyne, commonly called acetylene, is used as a fuel in welding blow torches. Many alkenes can take two geometric forms: cis or trans. The cis and trans forms of a given alkene are different isomers with different physical properties because there is a very high energy barrier to rotation about a double bond.
In the example below, the difference between cis and trans alkenes is readily apparent. Alkanes, alkenes, and alkynes are all classified as hydrocarbons , because they are composed solely of carbon and hydrogen atoms. The double and triple-bonded carbons in alkenes and alkynes have fewer hydrogen atoms bonded to them — they are thus referred to as unsaturated hydrocarbons. Aromatic groups are planar flat ring structures, and are widespread in nature.
When the carbon of an alkane is bonded to one or more halogens, the group is referred to as an alkyl halide or haloalkane. Chloroform is a useful solvent in the laboratory, and was one of the earlier anesthetic drugs used in surgery. Chlorodifluoromethane was used as a refrigerant and in aerosol sprays until the late twentieth century, but its use was discontinued after it was found to have harmful effects on the ozone layer. Bromoethane is a simple alkyl halide often used in organic synthesis.
Alkyl halides groups are quite rare in biomolecules. In the alcohol functional group, a carbon is single-bonded to an OH group the OH group, when it is part of a larger molecule, is referred to as a hydroxyl group.
Except for methanol, all alcohols can be classified as primary, secondary, or tertiary. In a primary alcohol, the carbon bonded to the OH group is also bonded to only one other carbon. In a secondary alcohol and tertiary alcohol, the carbon is bonded to two or three other carbons, respectively.
When the hydroxyl group is directly attached to an aromatic ring, the resulting group is called a phenol. The sulfur analog of an alcohol is called a thiol from the Greek thio , for sulfur. Note that the definition of a phenol states that the hydroxyl oxygen must be directly attached to one of the carbons of the aromatic ring.
The compound below, therefore, is not a phenol — it is a primary alcohol. The distinction is important, because there is a significant difference in the reactivity of alcohols and phenols. In an ether functional group, an oxygen is bonded to two carbons. Below is the structure of diethyl ether, a common laboratory solvent and also one of the first compounds to be used as an anesthetic during operations.
The sulfur analog of an ether is called a thioether or sulfide. Amines are characterized by nitrogen atoms with single bonds to hydrogen and carbon. Just as there are primary, secondary, and tertiary alcohols, there are primary, secondary, and tertiary amines. Ammonia is a special case with no carbon atoms.
One of the most important properties of amines is that they are basic, and are readily protonated to form ammonium cations. In the case where a nitrogen has four bonds to carbon which is somewhat unusual in biomolecules , it is called a quaternary ammonium ion. In alcohols, what matters is how many other carbons the alcohol carbon is bonded to, while in amines, what matters is how many carbons the nitrogen is bonded to.
Phosphate and its derivative functional groups are ubiquitous in biomolecules. Phosphate linked to a single organic group is called a phosphate ester ; when it has two links to organic groups it is called a phosphate diester. A linkage between two phosphates creates a phosphate anhydride.
There are a number of functional groups that contain a carbon-oxygen double bond, which is commonly referred to as a carbonyl. Ketones and aldehydes are two closely related carbonyl-based functional groups that react in very similar ways. In a ketone , the carbon atom of a carbonyl is bonded to two other carbons. In an aldehyde, the carbonyl carbon is bonded on one side to a hydrogen, and on the other side to a carbon. The exception to this definition is formaldehyde, in which the carbonyl carbon has bonds to two hydrogens.
The main member of this family is the carboxylic acid functional group, in which the carbonyl is bonded to a hydroxyl group. A single compound often contains several functional groups, particularly in biological organic chemistry. The six-carbon sugar molecules glucose and fructose, for example, contain aldehyde and ketone groups, respectively, and both contain five alcohol groups. The hormone testosterone, the amino acid phenylalanine, and the glycolysis metabolite dihydroxyacetone phosphate all contain multiple functional groups, as labeled below.
While not in any way a complete list, this section has covered most of the important functional groups that we will encounter in biochemistry. Identify the functional groups other than alkanes in the following organic compounds. State whether alcohols and amines are primary, secondary, or tertiary. Draw one example each of compounds fitting the descriptions below, using line structures.
Be sure to designate the location of all non-zero formal charges. All atoms should have complete octets phosphorus may exceed the octet rule. There are many possible correct answers for these, so be sure to check your structures with your instructor or tutor. Atoms can share pairs of valence electrons to obtain a valence shell octet.
This sharing of electrons is a covalent bond. A species formed from covalently bonded atoms is a molecule and is represented by a molecular formula , which gives the number of atoms of each type in the molecule. The two electrons shared in a covalent bond are called a bonding pair of electrons.
The electrons that do not participate in covalent bonds are called nonbonding pairs or lone pairs of electrons. A covalent bond consisting of one pair of shared electrons is called a single bond. Covalent bonds occur between nonmetal atoms.
Naming simple covalent compounds follows simple rules similar to those for ionic compounds. However, for covalent compounds, numerical prefixes are used as necessary to specify the number of atoms of each element in the compound.
In some cases, more than one pair of electrons is shared to satisfy the octet rule. Two pairs of electrons are shared by two atoms to make a double bond. Three pairs of atoms are shared to make a triple bond. Single, double, and triple covalent bonds may be represented by one, two, or three dashes, respectively, between the symbols of the atoms.
In the case of a coordinate covalent bond , one atom supplies both of the electrons and the other atom does not supply any of the electrons. The greater the electronegativity difference between the atoms involved in the covalent bond, the more polarity the bond displays. In comparison to ionic compounds, covalent molecules tend to have lower melting and boiling points, are less soluble in water, and are poor conductors of electricity.
These major differences are largely due to increased polarity of ionic bonds when compared with covalent bonds. Organic molecules can be represented in a number of different ways.
You should be able to recognize and draw out organic structures in each of these different structural representations: molecular formula, a displayed formula, a partially-condensed and fully-condensed structure, and line structures with wedges and dashes when appropriate. In addition, regions of an organic structure may represented by an R-group , to save time in structure recreation. This is especially useful when drawing a group of related compounds that only differ in one or two regions.
The differing regions of the molecule can be written out as R-groups to avoid having to redraw the entire molecule each time. Organic molecules can have isomer structures. Structural or constitutional isomers share the same molecular formula but the atoms within the structure are bonded together in a different orientation.
Stereoisomers have the same molecular formula and the atoms are also bonded together in the same order, however, the 3-dimentional arrangement of the atoms in space is different.
A special type of stereoisomer is called an enantiomer. Enantiomers are stereoisomers that are mirror images of eachother, but are not superimposable. Enantiomers have most of the same physical and chemical properties, however, since biological interactions depend on the 3-dimensional structure of molecules, enantiomers often have different biological activities. Molecules that are mirror images, but are not superimposable are said to have the property of chirality or handedness.
Carbon displays chirality when it has four different substituents on it. An -OH functional group on one molecule will tend to react similarly, although not identically, to an -OH group on another molecule. Functional groups are the key structural elements that define how organic molecules react, thus it is important to learn how to recognize common organic functional groups.
Chapter 5 materials have been adapted from the following creative commons resources unless otherwise noted:. Organic Chemistry Portal. Published under Creative Commons by-nc-sa 3. Poulsen, T. Molecules and Molecular Compounds. Clark, J. OpenStax CNX. Wikipedia, Ionic Compound. Physical and Theoretical Chemistry Libretexts. Soderberg, T. Organic Chemistry With a Biological Emphasis. Chapter 5: Covalent Bonds and Introduction to Organic Molecules This text is published under creative commons licensing, for referencing and adaptation, please click here.
We can represent the two individual hydrogen atoms as follows: In this situation neither hydrogen can reach the preferred duet state. In contrast, when two hydrogen atoms get close enough together to share their electrons, they can be represented as follows: By sharing their valence electrons, both hydrogen atoms now have two electrons in their respective valence shells.
For example, the Lewis diagrams of two separate hydrogen atoms are as follows: The Lewis diagram of two hydrogen atoms sharing electrons looks like this: This depiction of molecules is simplified further by using a dash to represent a covalent bond. The hydrogen molecule is then represented as follows: Remember that the dash, also referred to as a single bond, represents a pair of bonding electrons.
Two separate fluorine atoms have the following electron dot diagrams: Each fluorine atom contributes one valence electron, making a single bond and giving each atom a complete valence shell, which fulfills the octet rule: The circles show that each fluorine atom has eight electrons around it.
As with hydrogen, we can represent the fluorine molecule with a dash in place of the bonding electrons: Each fluorine atom has six electrons, or three pairs of electrons, that are not participating in the covalent bond. Consider a molecule composed of one hydrogen atom and one fluorine atom: Each atom needs one additional electron to complete its valence shell.
By each contributing one electron, they make the following molecule: In this molecule, the hydrogen atom does not have nonbonding electrons, while the fluorine atom has six nonbonding electrons three lone electron pairs.
For example, water, with two hydrogen atoms and one oxygen atom, and methane CH 4 , with one carbon atom and four hydrogen atoms, can be represented as follows: Atoms typically form a characteristic number of covalent bonds in compounds.
If each oxygen atom shares one electron with the carbon atom, we get the following: This does not give either the carbon or oxygen atoms a complete octet; The carbon atom only has six electrons in its valence shell and each oxygen atom only has seven electrons in its valence shell.
In carbon dioxide, a second electron from each oxygen atom is also shared with the central carbon atom, and the carbon atom shares one more electron with each oxygen atom: In this arrangement, the carbon atom shares four electrons two pairs with the oxygen atom on the left and four electrons with the oxygen atom on the right. Two pairs of electrons shared between two atoms make a double bond between the atoms, which is represented by a double dash: Some molecules contain triple bonds, covalent bonds in which three pairs of electrons are shared by two atoms.
A simple compound that has a triple bond is acetylene C 2 H 2 , whose Lewis diagram is as follows: Coordinate Covalent Bonds A coordinate bond also called a dative covalent bond is a covalent bond a shared pair of electrons in which both electrons come from the same atom.
The reaction between ammonia and hydrochloric acid If these colorless gases are allowed to mix, a thick white smoke of solid ammonium chloride is formed. Structural formulae and 3-dimensional models A structural formula shows how the various atoms are bonded, and can be more useful that only writing the molecular formula for a compound. Displayed formulae A displayed formula shows all the bonds in the molecule as individual lines with each atom written at the end of each line using its elemental abbreviation from the periodic table.
Condensed formulae For anything other than the most simple molecules, drawing a fully displayed formula can be cumbersome and take up too much space — especially all the carbon-hydrogen bonds. How to draw structural formulae in 3-dimensions There are occasions when it is important to be able to show the precise 3-D arrangement in parts of some molecules when using a structural representation. To do this, the bonds are shown using conventional symbols: For example, you might want to show the 3-D arrangement of the groups around the carbon which has the -OH group in 2-butanol.
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