Because polar molecules attract one another and nonpolar molecules are unable to squeeze between them, polar molecules do not mix well with nonpolar molecules. Because water molecules are polar, hydrocarbons are insoluble in water. Aldehydes with fewer than around five carbon atoms are soluble in water; however, aldehydes with more than five carbon atoms are insoluble due to the hydrocarbon part of their molecules. Hydrogen bonds form between the oxygen atom of the carbonyl group and hydrogen atoms of water molecules, resulting in the solubility of low-molecular-weight carbonyl compounds in water.
Is it possible for propanal to dissolve in water?
The solution phase is where almost all of the organic chemistry in this course takes place.
Nonpolar or slightly polar solvents such as toluene (methylbenzene), hexane, dichloromethane, or diethylether are commonly used in organic laboratories.
Many efforts have been made in recent years to adjust reaction conditions to allow the use of ‘greener’ (i.e., more ecologically friendly) solvents like water or ethanol, which are polar and capable of hydrogen bonding.
The solvent in chemical reactions that take place in the cytosolic area of a cell is, of course, water.
Any organic chemist must be aware of the factors that influence the solubility of different compounds in different solvents.
You’re certainly familiar with the solubility rule from general chemistry: ‘like dissolves like’ (and even before you took any chemistry at all, you probably observed at some point in your life that oil does not mix with water).
Let’s examine this ancient rule and use what we’ve learned about covalent and noncovalent bonding.
Assume you have a flask filled with water, as well as a variety of compounds to test to determine how effectively they dissolve in it.
Table salt, often known as sodium chloride, is the first material.
This ionic molecule dissolves rapidly in water, as you might expect if you’ve ever unintentionally swallowed a mouthful of water while swimming in the ocean. Why? Because water, as a very polar molecule, can form many ion-dipole interactions with both the sodium cation and the chloride anion, the energy released from these interactions is more than enough to compensate for the energy required to break up ion-ion interactions in the salt crystal and some water-water hydrogen bonds.
As a result, we now have individual sodium cations and chloride anions surrounded by water molecules in place of sodium chloride crystals – the salt is now in solution. Charged species, on the whole, dissolve easily in water, making them hydrophilic (water-loving).
Now we’ll attempt a chemical called biphenyl, which is a colorless crystalline material similar to sodium chloride (the two compounds, however, are easily distinguished by sight – the crystals seem extremely different).
In water, biphenyl does not dissolve at all.
What is the reason for this?
Because it has exclusively carbon-carbon and carbon-hydrogen bonds, it is a fairly non-polar molecule.
It can establish strong nonpolar van der Waals contacts with itself, but it cannot form significant attractive interactions with the highly polar solvent molecules.
As a result, the energy cost of breaking up biphenyl-to-biphenyl contacts in the solid is substantial, and new biphenyl-water interactions are few.
Nonpolar hydrocarbon molecules are extremely hydrophobic, therefore water is a lousy solvent for them (water-hating).
After that, you try a sequence of larger alcohol molecules, starting with methanol (1 carbon) and working your way up to octanol (8 carbons).
The smaller alcohols, such as methanol, ethanol, and propanol, dissolve quickly in water.
This is because water can create hydrogen bonds with these molecules’ hydroxyl groups, and the combined energy of forming these water-alcohol hydrogen bonds is more than enough to compensate for the energy lost when the alcohol-alcohol hydrogen connections are broken.
When you try butanol, though, you’ll find that it starts to build its own layer on top of the water as you add more and more.
Pentanol, hexanol, heptanol, and octanol are the longer-chain alcohols that are becoming progressively non-soluble.
What exactly is going on here?
Clearly, these bigger alcohols can form the same advantageous water-alcohol hydrogen bonds.
The distinction is that, in addition to their hydrophilic hydroxyl group, the bigger alcohols feature greater nonpolar, hydrophobic areas.
The hydrophobic effect begins to dominate the hydrophilic effect at about four or five carbons, and water solubility is lost.
Try dissolving glucose in water; it has six carbons like hexanol, but it also possesses five hydrogen-bonding, hydrophilic hydroxyl groups, as well as a sixth oxygen that can act as a hydrogen bond acceptor.
We’ve shifted the scales in favor of hydrophilicity, and we’ve discovered that glucose is extremely water soluble.
We discovered that ethanol is highly water soluble (if it weren’t, drinking beer or vodka would be a pain!) What about dimethyl ether, which is an ethanol constitutional isomer with an ether functional group instead of an alcohol functional group? Diethyl ether is found to be significantly less soluble in water. Does it have the ability to generate hydrogen bonds with water? The ether oxygen can, in fact, behave as a hydrogen-bond acceptor. The alcohol group, on the other hand, differs from the ether group in that it is both a hydrogen bond donor and acceptor.
As a result, compared to the ether, the alcohol is able to create more energetically advantageous interactions with the solvent, making it more soluble.
Another simple experiment that can be carried out in an organic laboratory (with suitable supervision).
If you try to dissolve benzoic acid crystals in room temperature water, you’ll discover that they’re not soluble.
Carboxylic acids, such as benzoic acid, are relatively weak acids, and hence exist largely in the acidic (protonated) state when added to pure water, as we will see in a later chapter while studying acid-base chemistry.
Acetic acid, on the other hand, is very soluble.
The small alcohol vs. large alcohol argument is simple to explain: the carboxylic acid group’s hydrogen-bonding, hydrophilic effect is strong enough to overcome the hydrophobic effect of a single methyl group on acetic acid, but not the larger hydrophobic effect of the 6-carbon benzene group on benzoic acid.
Now slowly pour some aqueous sodium hydroxide into the flask with the undissolved benzoic acid.
The benzoic acid begins to dissolve as the solvent grows more basic, until it is totally dissolved.
The benzoic acid is being changed to its conjugate base, benzoate, in this instance.
The hydrophilicity of the neutral carboxylic acid group was insufficient to compensate for the hydrophobicity of the benzene ring, but the carboxylate group, with its full negative charge, is significantly more hydrophilic.
As the highly hydrophilic anion part of the molecule drags the hydrophobic part of the molecule into solution, kicking and screaming (if a benzene ring can kick and scream), the balance is tipped in favor of water solubility.
Simply add enough hydrochloric acid to neutralize the solution and reprotonate the carboxylate to precipitate the benzoic acid back out of solution.
If you’re taking an organic chemistry course with a lab component, you’ll almost certainly undertake at least one experiment using this phenomena to separate an organic acid like benzoic acid from a hydrocarbon complex like biphenyl.
Similar reasons can be used to explain why different organic compounds are soluble in nonpolar or slightly polar solvents.
In general, the higher a molecule’s amount of charged and polar groups, the less soluble it is in solvents like hexane. For example, the ionic and very hydrophilic sodium chloride is insoluble in hexane, whereas the hydrophobic biphenyl is quite soluble in hexane.
Exercise 2.12: Vitamins are classed as either water-soluble or fat-soluble (fat is a non-polar, hydrophobic’solvent’).
Choose a category for each of the vitamins listed below.
Exercise 2.13: In pure water, both aniline and phenol are insoluble.
Explain your reasons for predicting the solubility of these two chemicals in 10% aqueous hydrochloric acid. Hint: aniline is basic in this situation, whereas phenol is not.
Is propanal a polar substance?
The total intermolecular forces are directly related to the boiling point trend of various substances. In general, the greater the overall intermolecular force acting on a substance, the higher the boiling point of that substance. The temperature at which a substance’s liquid phase vaporizes to become a gas is known as the boiling point. The intermolecular forces that hold the molecules together must be overcome in order to evaporate a liquid. The more energy necessary to overcome the forces, the higher the temperature required to overcome the forces, resulting in a higher boiling point.
Because the Molar Masses of all three compounds are comparable, the dispersion forces are also similar. The molecular polarity of the three compounds, however, differ. Propanal is a polar molecule with both dispersion and dipole-dipole forces, and propanol is a polar molecule with an OH link, thus all three types of forces apply. As a result, propanol has the most intermolecular forces, while butane has the least, which is the same order as their boiling points.
Why is propanal devoid of hydrogen bonds?
Propanol molecules have a hydrogen atom immediately attached to a NOF atom (oxygen in this example), allowing them to hydrogen bond with one another. Hydrogen bonding is not conceivable since the oxygen in the other three molecules is solely connected to a carbon atom. Van der Waals forces exist between all of the molecules, but propanol has the added benefit of hydrogen bonding, which is significantly stronger. The greater interactions between molecules necessitate more energy to break them apart, resulting in a higher boiling point for the substance. The temperature that corresponds to the energy required to break the intermolecular forces of attraction is known as the boiling point.
Aldehydes dissolve in water for a reason.
Small aldehydes and ketones are freely soluble in water, but as chain length increases, solubility decreases. For example, the common minor aldehydes and ketones, methanal, ethanal, and propanone, are miscible with water in all quantities. Although aldehydes and ketones cannot hydrogen link with other aldehydes or ketones, they can hydrogen bond with water molecules. A hydrogen bond can be established when one of the slightly positive hydrogen atoms in a water molecule is sufficiently attracted to one of the lone pairs on the oxygen atom of an aldehyde or ketone.
Between the aldehyde or ketone and the water molecules, there will be dispersion forces and dipole-dipole attractions. The energy released by forming these attractions contributes to the energy required to remove the water molecules from the aldehyde or ketone molecules before they may mix.
The hydrocarbon “tails” of the molecules (all the hydrocarbon portions apart from the carbonyl group) start to get in the way as chain lengths increase. They destroy the relatively strong hydrogen bonds between water molecules by shoving themselves between them without replacing them with anything as excellent. As a result, the process becomes less profitable in terms of energy, and solubility suffers.
What is propanol’s solubility?
The simple solubility rule that like dissolves like is a little more complicated in the case of alcohols, as it is in the case of many other biological compounds. Each alcohol is made up of a nonpolar carbon chain and an OH group (which is polar). The chemical formula for ethanol, for instance, is C2H5OH. Ethanol is a two-carbon compound with an OH group. The OH group is attracted to water because it is polar. The carbon chain, on the other hand, is repelled because it is nonpolar. The stronger of the two forces determines the solubility of alcohols.
The first three alcohols (methanol, ethanol, and propanol) are entirely miscible due to the strength of the OH group’s attraction. They dissolve in any amount of water. The solubility of alcohols begins to decline starting with the four-carbon butanol. Alcohols are deemed immiscible after the 7-carbon heptanol. The solubility of alcohol in water can be seen in the graph below. At 1atm and 25oC, the values are in mol/100g of H2O.
