Sometimes over there is an ext than one way to connect a provided group the atoms right into a molecular structure. Given the formula C2H6O, two different isomers room possible: methyl ether and ethanol. They have the exact same formula however different structures. The word, "isomer", is from the Greek, meaning "same things", introduce to their similar content. However, isomers frequently have an extremely different physical and biological properties.

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These two compounds have different physical properties, together you have the right to see in the table below.

compound melting pt, ° C boiling pt, ° C
dimethyl ether -138 -25
ethanol -117 78

Remember, common room temperature is about 25 °C. That means that at room temperature, dimethyl ether is a gas, since it is above its cook point. Ethanol, on the other hand, is a liquid at room temperature. Ethanol at that temperature is well above its freezing allude but has not yet got to its boiling point.

Why is that? What is the origin for the distinctions in physical properties between these two, comparable compounds, among which is a gas at room temperature while the other is a liquid? Both contain carbon-hydrogen and carbon-oxygen bonds. The ethanol also contains a carbon-carbon bond and an oxygen-hydrogen bond.

Certain bonds, or groups of bonds, confer certain behaviors ~ above the compounds in i beg your pardon they space found. For instance, when an OH team is discovered in an essential compound, it deserve to make the link moderately acidic. It likewise makes the link a little more likely to it is in a hard or liquid rather than a gas. Why?

The dominant intermolecular attraction in between ethanol molecule is hydrogen bonding. What we average is, although 2 ethanol molecules may be attractive by London dispersion forces and even by dipoles, both the those components pale in comparison to the attraction that arises from hydrogen bonding. Hydrogen bonding is the single most important factor holding two ethanol molecules together.

In dimethyl ether, ~ above the various other hand, hydrogen bonding is no a factor. In this molecule, the significant intermolecular attraction involves regular dipole moments. Since hydrogen bond are typically much more powerful attractions than plain dipole moments, a team of ethanol molecule is lot harder to different from each various other than a team of dimethyl ether molecules. The ethanol has a much higher boiling point.

Exercise (PageIndex1)

Compounds don"t really need to be isomers in stimulate to to compare them in this way. They just need to be similar enough the we have the right to reasonably compare them.

For example, propan-2-one or acetone, CH3(CO)CH3, has a boiling point of 56-57 °C whereas 2-propanol, CH3CH(OH)CH3, has a boiling allude near 83 °C. Explain the difference.


The hydrogen bonding in between 2-propanol molecule is a stronger interaction and is more complicated to conquer than the dipole-dipole interactions in between 2-propanone molecules. Thus, 2-propanone molecule can much more easily escape into the vapour phase.

Different collections of bonds that are frequently found in organic compounds are referred to as sensible groups. Functional groups influence the physics properties that the compounds in which they room found. They can additionally influence the biological activity of a compound; that is, lock can help determine even if it is a compound might be energetic as a specific type of drug, a hormone, or other species of compounds the regularly interact with biology in nature.

In the instance we looked at above, us were compare an alcohol come an ether. In one alcohol, over there is an OH group attached come a carbon. In an ether, over there is an O enclosed to 2 carbons. (There are exceptions to these definitions: if an OH team is attached come a carbonyl, a C=O group, the is no an alcohol but a carboxylic acid. If one oxygen is attached come a carbonyl and also to one more carbon, it is no an ether however an ester.) In general, we can say that, due to the fact that of the distinction in intermolecular attractions, an alcohol has a greater boiling suggest than a comparably-sized ether.

nature of products are very closely related to their framework Two isomeric compounds, written of precisely the same aspects in exactly the very same ratio, may have really different properties because of the different ways that the atom are associated within the compounds.

Another example of isomers is uncovered by looking at the formula C3H9N. This time there room three different structures the can occur with the given formula. All three isomers contain the "amine" useful group. One amine is simply a nitrogen linked to a mixture of carbons and hydrogens. Again, the 3 isomers have different properties, and also these differences are not just limited to melting points and also boiling points. What aspect do friend think is responsible for these differences.


compound melting pt, ° C boiling pt, ° C density, g/mL
trimethylamine -117 3 0.6356
N-methylethylamine not reported 37 0.688
n-propylamine -83 48 0.7173

The distinctions in melting points and also boiling point out are quite straightforward. The very first example own no potential because that hydrogen bonding. The second one does due to the fact that it has an N-H bond and also the third even an ext so, due to the fact that it contains two N-H bonds rather than one.

Unlike alcohols and ethers, whether or no an amine includes an N-H go not adjust the surname of the sensible group. That is still dubbed an amine. That might be an unfortunately artifact of history, because clearly an amine that have the right to hydrogen shortcut is a little different indigenous an amine the cannot, in simply the same way that an alcohol is various from one ether. To compensate because that that absence of distinction between amines, an additional adjective is offered to explain them. An amine in i beg your pardon the nitrogen is enclosed to just one carbon and two hydrogens is referred to as a "primary amine" (primary method one, together in one carbon attachment). An amine in i m sorry the nitrogen is fastened to 2 carbons and also one hydrogens is referred to as a "secondary amine" (secondary way attachment to 2 carbons). One amine in which the nitrogen is enclosed to 3 carbons and also no hydrogens is dubbed a "tertiary amine" (attachment to three carbons).

Isomers have actually a variety of properties the are various from the nature of various other members of their group. Not every one of these properties have the right to be easily defined by looking at different structures, but some can. The density of a material is the ratio of its load to its volume (in an ext "scientific" language: mass/volume). A product with a higher density than one more can be believed of as having actually the same weight, however shrunken to a smaller volume. Alternatively, it can be assumed of as having the exact same volume, yet packed complete of much more mass.

Note that the isomers in the table over have different densities. The densities in the table refer to liquid densities. In spite of their similarities, the 3 compounds in the fluid state load together a small differently. That, too, is a an outcome of structural differences.

Exercise (PageIndex2)

Explain the differences between the densities in the table above. The worths in the table all describe the products in your liquid states.


There is hydrogen bonding between molecules of N-methylethylamine, yet not between molecules that triethylamine, i m sorry are organized together by relatively weak dipoles. Trimethylamine molecules space not organized together as tightly as the N-methylethylamine molecules, which as such pack much more densely. Top top the various other hand, n-propylamine contains two N-H bond, contrasted to just one in N-methylethylamine. Consequently, n-propylamine has a better propensity to kind hydrogen bonds, and also it packs together even an ext densley 보다 the N-methylethylamine.

There are various other properties that can be compared and also explained based on the structures of compounds. Vapour pressure is carefully related to boiling point. It describes the push exerted by molecule escaping indigenous a liquid right into the vapour phase. The more easily molecule can acquire away native each other in the liquid and also escape right into the vapour phase over the liquid, the higher the vapour pressure. The higher the vapour pressure, the an ext easily the fluid reaches the cook point. In a basic sense, we deserve to think of strong attractions between molecules holding castle in the liquid phase and also lowering your vapour pressure.

Viscosity describes the resistance to flow in a liquid. The higher the viscosity the a liquid, the an ext slowly it flows. Maple syrup is much more viscous than water, however honey is an ext viscous 보다 maple syrup. In a streamlined way, we have the right to think of molecules clinging come their next-door neighbors as they flow past every other, hindering flow and also increasing viscosity. Thus, the intermolecular attractions between molecules in a liquid influence its viscosity.

It"s a small like a group of friends rushing under a crowded street in Manhattan. If lock all organize hands, they don"t make as rapid development as they would certainly individually. Individually, every member that the group is able to step aside, dart forward and also get previous the oncoming pedestrians. Clinging together, they all need to turn, come to a halt and also wait for people to permit them past.

Surface tension is a measure of how strongly a fluid resists outside forces. You can think of the as exactly how tightly the molecules in a product cling to each various other when offered the opportunity to cling come something rather instead.

The most familiar picture of surface anxiety is a autumn of water. Water forms drops since the molecule in the water cling to each other more strongly 보다 they perform to the molecule in the air. Those are greatly nitrogen molecules, N2, through a far-ranging fraction of oxygen molecules, O2. The molecules in the wait are fairly non-polar, due to the fact that they are made of the exact same kinds the atoms: 2 nitrogens, for instance. There is no electronegativity difference that would impart a dipole to the molecule. That"s in stark comparison to a highly polar water molecule; it"s tough to uncover molecules an ext polar than that. If a water molecule to be to communicate with a molecule of dinitrogen, it would certainly be trading in some solid hydrogen binding with other molecules. The wouldn"t be precious it.

Exercise (PageIndex5)

Explain the distinctions in the viscosities the the adhering to compounds (listed in centiPoise, cP, or milliPascal-seconds, mPa.s).

diethyl ether, CH3CH2OCH2CH3 (0.22) vs. 2-butanone, CH3(CO)CH2CH3 (0.41) decane, CH3CH2CH2CH2CH2CH2CH2CH2CH2CH3 (0.70) vs hexane, CH3CH2CH2CH2CH2CH3 (0.26) diethyl ether, CH3CH2OCH2CH3 (0.22) vs. 1-butanol, CH3CH2CH2CH2OH (2.54) Answer a:

The C=O link 2-butanone has a an extensive dipole, yet the dipoles in ether don"t add up to really much; they are smaller to start with and also they partly cancel out via vectorial addition. The 2-butanone molecule cling together much more strongly, leading to a higher viscosity.

Answer b:

Both molecule are restricted to relatively weak London dispersion interactions. Due to the fact that decane develops a much longer chain than hexane, there will be a slightly higher interaction in between decane molecule than between hexane molecules. Decane thus has actually the greater viscosity. Later, if you examine macromolecules, girlfriend will view that in very, very long chains, "entanglement" becomes a aspect as well. Just as the surname implies, really long chains obtain tangled increase together and also have a challenging time moving past each other; that"s why materials like cooking oil and motor oil are so viscous.

Answer c:

The 1-butanol have the right to hydrogen link together, but the ether only has weak dipole-dipole interactions. The 1-butanol is therefore more viscous.

You don"t need to have data in prior of you to compare two compounds" properties. At least to some extent, you may be able to look at two comparable structures and make an educated guess about which one will certainly melt or evaporate an ext readily than the other, detailed they have various functional teams that you have the right to compare. Understanding something around structure has explanatory value; girlfriend can define differences based upon structure. It additionally has predictive value: you can predict differences based on structure.

See more: Why Is The Atomic Mass Of Hydrogen 1.008 And Not 1? Why Is The Atomic Weight Of Hydrogen 1008 And Not

Exercise (PageIndex9)

In every of the complying with questions, you have a selection between A and also B. Choose which choice in which stronger attractions space present.